3.

SIGNALLING SYSTEMS

3.1 TYPES OF SIGNALLING SYSTEMS:

Signalling Systems can be broadly categorized in following two Categories:

(i) Two Aspect Signalling Systems.

(ii) Multi Aspect Signalling Systems.

3.2 TWO ASPECT SIGNALLING SYSTEMS:

Two Aspect Lower Quadrant (2-ALQ) Semaphore type Signalling was the earliest type of
Signalling introduced on the Railways. This type of Signalling employs mechanically
operated 'Semaphore' Signals of ‘Lower Quadrant: Two Aspect' type. Till the
beginning of the fifties, bulk of the sections on Indian Railways had this type of
Signalling.

'Two Aspect’ Signalling, whether Semaphore type or Colour Light type is not suitable
for sections having large number of Trains running at high speed. This is for the basic
reason that the Stop Signals, in this type of Signalling are not pre-warned by the Signals
in rear with the result that there is always a possibility of a Driver overshooting a Signal
displaying 'Danger' Aspect. Drivers are, therefore, not confidant enough to approach
the Block Stations, at high speed, with this System of Signalling. They reduce the speed
of their Trains while approaching a Station to keep the Train under perfect control and
this adversely affects the Line Capacity of the section.

3.3 MULTI-ASPECT SIGNALLING SYSTEMS:

In Multi Aspect Signalling Systems, employing mechanically operated UQ Semaphore

Signals, the Signals are capable of displaying three Aspects instead of two as is in the
case of 2-A Signalling. 'Distant' Signal in this System of Upper Quadrant Signalling not
only repeats the Aspect of 'Home' Signal ahead but also gives an Indication to the Driver
as to whether he is going to be stopped at the 'Home' Signal itself or being received on
the Main line or Loop line. Run through Indication in this type of Signalling is conveyed
to the Driver at 'Home' Signal. Multi Aspect Signalling is, thus, free from the deficiencies
inherent in the Two Aspect Signalling.

As MA Signalling provides added Information to the Driver at the 'Distant' Signal itself,
Driver can suitably Control the speed of his Train without any adverse effect on the Line
Capacity of the section. When the Train is being received on the Main line, it may run at
normal speed up to the 'Home', as Normal Braking Distance (NBD) is available between
Home and next Stop Signal viz. the Starter Signal. When a Train is to be received on
Loop line, the speed is reduced just at the time of passing the 'Distant' Signal, to the
extent necessary for negotiating the Turnover connecting Loop line to Main line. When a
Train is to be stopped at the 'Home' Signal, Brakes can be applied at the foot of 'Distant'
Signal, the most appropriate point, as NBD is available between the 'Distant' and 'Home'
Signal. It is, thus, possible to Control the speed of the Train within the Station Limits in
such a manner that unnecessary speed reductions and resultant loss in Line Capacity
are avoided. From Safety and Speed point of view, MA Signalling is, therefore, definitely
superior to 2-A Signalling.
In MA Signalling, as the Signals repeat the Aspects of Signal in advance, the Divers
move confidently in the section. They are not tense in locating the First Stop Signal
(FSS), while on run. Problem of uncertainties of Sighting Distance (SD) gets completely
eliminated and for Design purposes, Sighting Distance (SD) to be provided for a Multi
Aspect Signal can be taken as Zero. Driver need not unduly strain himself to pick up the
Aspect of a Signal from a distance and instead can pick up the Aspect comfortably when
he is close to the Signal. If the 'Distant' Signal, the first Signal encountered by the
Driver is displaying 'Proceed' (Green) Aspect, he proceeds at normal speed. If the
'Distant' Signal is exhibiting 'Attention' (Double Yellow) Aspect, he knows that he has to
Control the speed to negotiate Turnover to enter to Loop line and when 'Distant' is
showing 'Caution' (Yellow), he should apply brakes to bring his Train to stop at 'Home'.
When he comes to 'Home' Signal at normal speed after passing 'Distant' and if Home is
displaying 'Proceed' (Grren) Aspect, he knows that he is running through the Station and
he, therefore, does not need to reduce the speed of his Train but if 'Home' is at 'Caution'
(Yellow), he makes a Normal Brake Application to stop the Train at 'Starter' Signal.

3.4 MULTI ASPECT COLOUR LIGHT SIGNALLING (MACLS):

Multi Aspect Colour Light Signalling uses electrically lit Colour Light Signals, which
because of their powerful and penetrating Light provide better visibility to the Drivers as
compared to Semaphore Signals. This type of Signalling (MACLS), thus, ensures better
identification of Signal Aspects even under adverse weather conditions and thereby
provides increased Safety in Train Operation.

Figure 3.1 shows a typical Yard Layout, equipped with MACL Signalling Scheme.

In view of inherent short comings of 2-Aspect Signalling, the Indian Railways have been
pursuing a policy of replacement in a phased manner, of 2-A Signalling on the Trunk
Routes and Main line Routes, by Multi-Aspect Signalling wherein each one of the fixed
running Signal is of the type 'Multi-Aspect' and each of the Stop Signal is 'pre-warned' by
the Aspect of Signal in rear.

3.5 TYPES OF SIGNALS USED IN MULTI ASPECT COLOUR

LIGHT SIGNALLING TERRITORIES:

(i) STOP Signal:

(ii) DISTANT Signal:

Distant Signal is a Permissive Signal.

3.5.1 Aspects & Indications:

Aspects & Indications of STOP & DISTANT Signals in MACL Signalling Territories are
detailed in the following Table:

Aspect Indication

3-A STOP Signal:

STOP Red Light Stop Dead.

 CAUTION Yellow Light Proceed and be prepared
to stop at the next Stop
Signal.

PROCEED Green Light Proceed at maximum

permissible speed.

4-A STOP Signal:

STOP Red Light Stop Dead.

CAUTION Yellow Light Proceed and be prepared

to stop at the next Stop
Signal.

ATTENTION Two Yellow Lights in Proceed and be prepared

vertical alignment, one to pass the next Signal at
above the other (Double Caution.
Yellow).

PROCEED Green Light Proceed at maximum

permissible speed.

DISTANT Signal:

CAUTION Yellow Light Proceed and be prepared

to stop at the next Stop
Signal.

ATTENTION Two Yellow Lights in Proceed and be prepared

vertical alignment, one to pass the next Signal at
above the other (Double Caution.
Yellow).

PROCEED Green Light Proceed at Maximum

Permissible Speed.

Aspects & Indications of STOP &

DISTANT Signals in MACL Signalling

Figure 3.2 shows a 3-Aspect STOP Signal and Figure 3.3 a 4-Aspect STOP Signal.
Figure 3.4 shows (Inner) Distant Signal and Figure 3.5 (Outer) Distant, in Territories
where two Distant Signals are provided. This is the case on High-speed Routes on
Indian Railways.

3.6 SUPERIORITY OF MULTI ASPECT SIGNALLING OVER TWO

ASPECT SIGNALLING:

Stop Signals, in 2-A Signalling are not pre-warned by the Signals in rear with the result
that there is always a possibility of a Driver overshooting a Signal displaying 'Danger'
Aspect. Drivers are, therefore, not confidant enough to approach the Block Stations, at
high speed, with this System of Signalling. They reduce the speed of their Trains while
approaching a Station to keep the Train under perfect control and this adversely affects
the Line Capacity of the section. MA Signalling eliminates this primer lacuna in 2-A
Signalling.

In MA Signalling, as the Signals repeat the Aspects of Signal in advance, the Divers
move confidently in the section. They are not tense in locating the First Stop Signal
(FSS), while on run, as is the case in 2-A Signalling Territories. Problem of uncertainties
of Sighting Distance (SD) gets completely eliminated and for Design purposes, Sighting
Distance (SD) to be provided for a Multi Aspect Signal can be taken as Zero. Driver
need not unduly strain himself to pick up the Aspect of a Signal from a distance and
instead can pick up the Aspect comfortably when he is close to the Signal. If the
'Distant' Signal, the first Signal encountered by the Driver in MA Signalling is displaying
'Proceed' (Green) Aspect, he proceeds at normal speed. If the 'Distant' Signal is
exhibiting 'Attention' (Double Yellow) Aspect, he knows that he has to Control the speed
to negotiate Turnover to enter to Loop line and when 'Distant' is showing 'Caution'
(Yellow), he should apply brakes to bring his Train to stop at 'Home'. When he comes to
'Home' Signal at normal speed after passing 'Distant' and if Home is displaying 'Proceed'
(Grren) Aspect, he knows that he is running through the Station and he, therefore, does
not need to reduce the speed of his Train but if 'Home' is at ‘Caution’ (Yellow), he makes
a Normal Brake Application to stop the Train at 'Starter' Signal.

MA Signalling is, thus, distinctly suitable from the view point of ensuring Safety in Train
Operations as also permitting higher Speeds on the section.

Viewing from Speed point of view, MACLS is a better Speed Signalling for the Driver as
he is always pre-warned for the Aspect of Signal located in advance. This facilitates
better Speed Control of the Train in such a manner that unnecessary speed reductions
are eliminated. Result is increased Line Capacity obtainable on the section.

Viewing from the view point of Safety in Train Operations, in MA Signalling Sighting
Distance of Colour Light Signals can be safely taken as zero due to the fact that Aspect
Control Charts are designed in a way that NBD is always available to the Driver between
'Y' and 'R' Aspects. Drivers can thus react to the Aspect of Signal, even at the foot of
the Signal post. From Safety point of view, also, MACLS is a better Signalling.

3.7 ASPECT CONTROL CHARTS:

Figure 3.6 provides for MACL Signalling Scheme at a typical 'B' Class Station on a
Double line section. Signalling arrangements include two Distants, Home with Route
Indicator, Loop & Main Line Starters and Advance Starter Signals for either direction. 2 nd
Distant Signal has been added, presuming that the Station is on high-speed Trunk /
Main line Route.

Aspect Control Sequence for various movements in to the Station is given in the
following Table:

Train Movement 2nd Distant Distant Home Starter Signal Advanced

particulars Signal Signal Signal Starter
MS LS Signal
 (a) Stopping YY Y R
outside the
station at
Home Signal.

(b) Reception on G YY YwU R

Loop Line.
(Yellow
with
Route)

(c) Reception on G G Y R
Main Line.

(d) Run through on G G G G G

Main Line.

Aspect Control Sequence at an Installation equipped with MACL Signalling

As seen from the Aspect Control Chart in this Scheme of Signalling also, the Aspect of
each Signal is 'pre-warned' by the Aspect of Signal in rear.

2nd Distant Signal, added on High-speed Trunk / Main Line Routes provides for
Reception Status Information much in advance, thereby infusing higher level of
confidence in Drivers of Mail / Express / Super Fast Trains and in turn helps in improving
the Safety in Train Operation. A study of Control Charts conducted on one of the busy
section of one of the Zonal Railways on Indian Railways, before and after commissioning
of 2nd Distant Signal, has revealed a reduction in running time for Mail / Express / Super
Fast trains by 4% to 5%. Other advantages of 2nd Distant Signal, viewed from Driver's
point of view are summarised below:

(i) After commissioning of 2nd Distant Signal, Drivers feel more confident in
running at permitted full speed.

(ii) Signals are at regular intervals, which help Drivers to be more alert, and it
breaks their monotony.

(iii) Even though Signals and Aspects are more, they convey clear
meaning from an Adequate Distance.

(iv) Automatic Signalling at a later date can be introduced with some

incremental inputs only.

3.8 PANEL OPERATION OF SIGNALLING GEARS:

Panel Operation of Signalling Gears at a Station, besides providing Centralised

Operation, is also significantly advantageous in comparison with so far prevalent
Operation of Signals from an Interlocked Frame:

(i) Operating the levers in a Lever Frame requires a lot of physical effort on
the part of Cabinman. Also, there could be a limit to the number of levers
 that could be operated in a given time period without imposing undue
strain on the Cabinman. The strain is both physical and mental. There is
also a necessity to pull the Levers in a certain specific order, demanding
concentration and greater knowledge on the part of the Cabinman.
These factors together with the traffic density requiring frequent Shunting
operations in big busy Stations limit the number of levers that could be
operated by one Cabinman and often more than one Cabinman had to be
employed in big installations, which further creates problems of
coordination.

(ii) In Panel / Route Relay Interlocking, Operation of Signals and Points is

electrically through Push Buttons, which avoids delays to the Train
running and thus enable an increased traffic density to be handled at a
Station. Panel operation significantly reduces the time ‘To’ for operation of
Signalling gears.

3.9 SIGNALLING ARRANGEMENTS AT CLASS ‘A’, ‘B’ & ‘C’

STATIONS:

In Absolute Block System, the Block Stations are of three types – Class ‘A’, Class ‘B’ &
Class ‘C’.

(i) Class ‘A’ Stations are those where Line Clear may not be given for a
Train unless the Line on which it is intended to receive the Train is Clear
for at least 400 metres beyond the Home Signal, or up to the Starter. It
would mean that at such Stations, a second Train can be permitted to
enter into a Block section only after the previous Train has cleared the
Starter of the next Station.

Following compliment of Signals are minimum to be provided at a Class

‘A’ Station:

A Warner, a Home and a Starter.

(ii) Class ‘B’ Stations are Stations where Line Clear may be given for a
Train before the Line has been cleared for the Reception of the Train
within the Station section. On such Stations, Shunting can be permitted in
the face of an Approaching Train. A Shunting Limit Board (SLB), Block
Section Limit (BSL) Board or an Advance Starter demarcates the point up
to which the Shunting can be permitted.

Following Compliment of Signals are to be provided at a Class ’B’ Station

under different types of Signalling:

2-Aspect Signalling:

Single Line: An Outer & a Home + A Warner if Trains run

through at a Speed exceeding 50 Kmph. + Advance
Starter or SLB where Shunting in the face of an
Approaching Train is permitted.

Shunting Limit Board (SLB) or Advance Starter shall be

placed at such Shunting Distance from the Home Signal or
 the outermost Facing Points as local conditions may
require, provided the Distance between the SLB or the
Advance Starter and the opposing FSS is never less than
400 metres.

Double Line: An Outer, a Home, and a Starter + a Warner

if Trains run through at a Speed exceeding 50 Kmph.

Multi Aspect Signalling:

A Distant, a Home, and a Starter for each Direction.

On Single Lines, where Shunting in the face of an

Approaching Train is permitted, Shunting Limit Board
(SLB) or Advance Starter shall be placed at such Shunting
Distance from the Home Signal or the outermost Facing
Points as local conditions may require, provided the
Distance between the SLB or the Advance Starter and the
opposing FSS is never less than 180 metres.

On Double Lines, where there are no Points or the

outermost Points at approaching end are Trailing, a Block
Section Limit Board (BSLB) shall be provided and shall be
placed at a distance of 180 metres in advance of the Home
Signal and shall protect the Fouling Mark of the outermost
Trailing Points, if any. The location of such Board shall
mark the limit of the Block section at such Stations.

(iii) Class ‘C’ Stations, also called the Block Huts, are those where Line
Clear for a Train may not be given for a Train, unless the whole of the last
preceding Train has passed ‘Complete’ at least 400 metres beyond the
Home Signal, and is continuing its journey.

Class ‘C’ Stations also include Intermediate Block Huts (IBH).

Following Compliment of Signals are minimum to be provided at a Class

’C’ Station under different types of Signalling:

2-Aspect Signalling:

A Warner & a Home.

Multi Aspect Signalling:

A Distant & a Home.

If we closely examine the Class ‘B’ working in Multi Aspect Signalling Territories, it can
be seen that it provides for both ‘A’ Class & ‘B’ Class working of 2-Aspect Signalling.

From the Definitions of Class ‘A’ & Class ‘B’ Stations, following important inferences can
be drawn:
(i) At Class ‘A’ Stations, the First Signal encountered by the Driver is a
Permissive Signal i.e. the Warner in 2-A Signalling Territories.

(ii) At Class ‘B’ Stations:

(a) Line Clear may be given for a Train before the line has
been cleared for the reception of the Train within the
Station section, as Block Overlap is available beyond the
FSS, without taking in account the Berthing Line portion of
the Station Yard i.e. while a Train is berthed on the
Berthing Line, there is no infringement to Block Section.

(b) Shunting can be permitted in the face of an approaching

Train. A Shunting Limit Board (SLB), Block Section Limit
(BSL) Board or an Advance Starter demarcates the point
up to which the shunting can be permitted.

Now Consider the following Figures:

Figure 3.6: Layout of Class ‘B’ Station equipped with Multi Aspect
Colour Light Signalling – Double Line Section.

Figure 3.7: Layout of Class ‘A’ Station equipped with 2-Aspect Colour
Light Signalling – Double Line Section.

Figure 3.8: Layout of Class ‘B’ Station equipped with 2-Aspect Colour
Light Sgnalling – Double Line Section.

Layout in Figure 3.7, of Class ’A’ Station, equipped with 2-Aspect Signalling has the First Signal
as Warner, which is a Permissive Signal. Drivers approach such Stations with confidence as
they are well aware that NBD is available between First Permissive Signal and the FSS.

Layout in Figure 3.8, of Class ’B’ Station, equipped with 2-Aspect Signalling has the First Signal
as Outer / Warner Combination. Outer being a Stop Signal, Drivers have to move with Caution
and look out for the Signal to react in time to apply brakes and stop at it, should the Signal be
exhibiting a Danger Aspect. The Signalling, however, permits granting of Line Clear before the
Line has been cleared for the reception of the Train within the Station section, as Block Overlap
of 400 ‘m’ is available between Outer and Home, Outer being the FSS. Signalling also permits
Shunting in the face of an approaching Train as Block section and Station section are clearly
demarcated.

Layout shown in Figure 3.6, of Class ‘B’ Station, equipped with MACL Signalling provides both
the features of Class ‘A’ and Class ‘B’ working of 2-Aspect Signalling:

(i) First Signal to be encountered by the Driver is Distant Signal, which is a

Permissive Signal.

(ii) Line Clear can be granted to a Train, when another Train is standing on the
Berthing Line. Notice the Block Overlap of 180 ‘m’ having been provided between
Home and Facing Point / Fouling Mark of the Emergency Crossover.

(iii) Shunting can be performed up to BSLB in the face of an approaching Train or up

to the Advanced Starter.
3.10 CONSIDERATIONS FOLLOWED IN LOCATION & PLACEMENT
OF DIFFERENT TYPES OF SIGNALS:

Signals should be so located and aligned as to:

(i) Display the best possible view of their Aspects to the Drivers of
approaching Trains. Signal should also be made to be continuously
visible to the Driver.

(ii) Avoid the possibility of the Aspect of one Signal being mistaken for the
Aspect of another.

(iii) Avoid confusion between the lights of Running Signals and the lights of
Subsidiary Signals or any other lights in the Yard.

(iv) Afford the Sighting Distance (SD) required of them. Signal Engineering
Manual provides for 200 metres of Sighting Distance for all Stop Signals.

(v) Work them efficiently.

(vi) Not infringe the schedule of Dimensions.

Signals shall normally be located on the left of or above the line (in the case of Gantry
Mounted Signals) to which they apply, unless authorised by approved Special
Instructions.

3.10.1 Location of Home Signal:

(A) 2-Aspect Home:

(i) Close to the first set of facing Points.

(ii) If it is necessary to increase the distance between the

Home and Facing Points, beyond 180 meters, other arrangements
for holding the Route such as Holding Bar with Succession
Locking / Fouling Bar / Track circuits / Station Master’s Route
Control, are to be essentially made.

(B) Home in Multi Aspect Signalling Territories:

(i) Home in MA Signalling Territories is the First Stop Signal and in

order to obtain maximum Operational Facilities on Single lines, is
located at 300 meters (Block Overlap (BO) = 180 metres + Signal
Overlap (SO) = 120 metres) from the Facing Points or the nearest
Fouling Mark.

(ii) On Double Lines, MA Home

may be located Block Overlap (180 metres) in rear of the Facing
Points or nearest Fouling Mark.

3.10.2 Location of Outer & Warner Signals:

(A) Outer Signal:

(i) Outer signal is the First Stop Signal (FSS) in 2-A Signalling
Territories. The minimum distance, from Operational Facilities
viewpoint, between Outer and Home, on Single lines, should be:

(a) At least 580 metres (400 m + 180 m) so that

Block Overlap (BO) for one Train and Signal
Overlap (SO) for an opposing Train, approaching a
Station simultaneously can be provided separately.

(b) If this distance is less than 400 m + 180 m,

the first of the two simultaneously Approaching
Trains has to be stopped at the respective Outer
Signal.

(ii) On Single lines, Block Overlap of 400 metres is to be necessarily

made available between Outer and opposing Advance Starter or
Shunting Limit Board.

(iii) On Double lines, a distance of 400 metres between the Outer and
Home would be an adequate Operational Requirement.

(iv) Outer, if it is FSS, should be located Normal Braking Distance

(NBD) plus Reaction Time (RT) in rear of the Home. Where it is
not possible to provide NBD, at least Emergency Braking Distance
(EBD) should be provided between the two Signals.

(B) Warner Signal:

At a location of ‘NBD + RD’ in rear of Stop Signal in its advance. Where it

is not possible to provide Normal Braking Distance, at least Emergency
Braking Distance (EBD) is to be maintained between Warner and Stop
Signal in its advance.

3.10.3 Location of Starter & Advanced Starter Signals:

(A) Starter Signal:

(i) At the Leaving end of Departure line, protecting the Fouling Mark

or

At a distance of SO in rear of the Fouling mark to provide

Operational Flexibility but this would reduce the length of the
Berthing line.

(ii) Distance between Home & Starter should not be less than BO i.e.
400 metres in case of ‘A’ Class Stations and 180 metres in case of
MA Signalling Stations.

(B) Advanced Starter Signal:

 (i) On Single line, Advance Starter is usually placed 180 metres from
the outermost trailing points so that it may be used to denote the
end of SO for Home signal as also to indicate the Shunting Limit.
This distance may be increased beyond 180 metres, if required for
Shunting or other purposes provided the Outer in ‘B’ Class and
Home in MACLS, can be located at a distance of at least BO from
the Advanced Starter.

(ii) On Double Line, Advance Starter is usually placed at a location

SO in advance of the Starter Signal.

3.11 STATION LAYOUTS WITH DIFFERENT COMPLEMENT OF

SIGNALLING SYSTEMS:

(i) Standard-III MACL Signalling with End Cabins: Figure 3.9. (Refer
to Figure 3.10 for a Photograph of a Catch Handle type Lever
Frame used in Multi Cabin Signalling System to operate Signalling
Gears. Figures 3.11 to 3.13 provide Photographs of Signalling
Yards operated through Multiple Numbers of Lever Frames
installed in Cabins. The Signals are of Colour Light type and
Points are connected via Rodding Runs to the Levers in the Lever
Frame).
(ii) Standard-III MACL Signalling with End Panels: Figure 3.14.
(iii) Standard-III MACL Signalling with Central Panel: Figure 3.15.
(iv) Standard-III MACL Signalling with Central Panel & Double Distant
Signals: Figure 3.16. Figure 3.17 shows a Photograph of a Relay
Interlocked Station operated through a Central Control Panel.

3.12 SIMULTANEOUS RECEPTION & DISPATCH FACILITIES:

In any Interlocked Yard, the Interlocking has to conform to certain basic requirements.
Which include essential conditions for taking ‘OFF’ of a Signal for an intended movement
and holding the Route to be traversed by the Train on the said Signal, until the
movement is completed. The Interlocking also prohibits taking ‘OFF’ of any two fixed
signals, at the same time, which can lead to any conflicting movements.

General Rules provide for conditions for taking ‘OFF’ of Signals for more than one Train
at a time:

‘When two or more Trains are approaching simultaneously from any

direction, the Signals for one Train only shall be taken ‘OFF’, other
necessary Signals being kept at ‘ON’, until the Train for which the Signals
have been taken ‘OFF’ has come to a stand at the Station, or has cleared
the Station, and the Signals so taken ‘OFF’ for the said Train have been
put back to ‘ON’.

There is, however, an exception permitted to this Rule which lays down that where under
Approved Special Instructions, the Interlocking or layout of the Yard renders it safe,
Signals for more than one Train may be taken ‘OFF’ at the same time.

Taking ‘OFF’ of Signals for different Trains for Reception / Departure at the same time is
called ‘Simultaneous Reception & Dispatch of Trains’.
Provision of simultaneous Reception & Dispatch facilities enable:

(i) Two opposite Trains on Single line sections to be received or crossed

without either of them stopping at a Reception Signal, and

(ii) Two Trains running in same direction on Double lines to be received and
departed on / from Reception lines at a Station.

Simultaneous Reception will only be possible if for each and every one of the
approaching Trains, the line on which the train is to be received is clear up to the point
where the Train is to come to a stand and an Overlap distance in advance of it.

Figure 3.18 shows typical arrangements for simultaneous Reception and Dispatch
facilities on Single and Double Line sections. These include:

(i) Provision of Sand Humps, of approved Design, on Loop Lines. The Rules
recognise the use of a short Sand Hump siding in lieu the distance
specified for the Signal Overlap.

The points of the Sand Hump siding when set for the Sand Hump also
serve as a good method of isolating the adjacent connected Line.

Sand Hump sidings are, however, more expensive in initial cost and
subsequent maintenance.

Earlier layouts also provided Sand Humps on Main Line but that practice
has now been done away with as the Main line is meant for high speed
operations.

(ii) Sidings on Loop Lines: These Sidings can not be used for Stabling and if
used, Trap Points shall have to be provided for Isolation purposes and the
distance up to the trap Point shall be equal to the Overlap required.

(iii) Provision of Calling-on Signals on either side Home Signals.

(iv) Configuring Signal Overlaps (SO) on Reception Lines. This arrangement,

however, is associated with lacunae in as much as that Berthing portion
on Reception Lines does get reduced, which may cause operational
bottlenecks, while handling longer Passenger & Goods Rakes.

Simultaneous Reception & Dispatch facilities reduce the delays, involved in Reception of
a Train as the Reception Signals for the two opposite Trains on Single lines and
Departure Signals for a Train stationed at a Station & Reception Signal for another Train
in the Block section in rear on Double lines, can be lowered simultaneously, thereby
resulting in saving of time for the Train in question to the extent of 8 to 10 minutes.

3.13 ISOLATION:

The term ‘Isolation’ denotes the condition in which a line for a particular movement is
separated from all adjoining lines connected to it in such a manner that the Isolated Line
cannot be fouled or interfered with by any movement taking place on the adjoining lines.
Isolation is compulsory in the following cases:
(i) A line on which Train movements at speeds higher than 50 Kmph are
permitted should be isolated from all connected lines.

(ii) Passenger lines should be isolated from all connected Goods lines and
Sidings, whatever the speed may be.

(iii) Passenger lines where Trains run at a speed of 50 Kmph or below need
not be isolated from other such Passenger running lines.

(iv) The Isolation of Goods Reception lines from Sidings is considered

desirable.

(v) One Goods Reception Line need not be isolated from other Goods
Reception lines, when dealing with speeds of 50 Kmph and less.

(vi) Provisions of sub-para (i) do not apply to:

(a) Junctions where two Block section lines meet at the same
end of a Station and the System of Block working with
adjacent Stations on both lines is done by one of the
approved means and the junction is equipped with full
complement of Signals.

At such Junctions, the FSS on Single line sections shall be

placed at an Adequate Distance (AD) from the outermost
Facing Points / Fouling Mark, the AD not being less than
the total of the Adequate Distances required in regard to
condition for granting of Line Clear and in regard to
condition for taking ‘OFF’ of Home Signal. On Double line
section, the FSS shall be placed at an AD from the
outermost facing Point / Fouling Mark, this distance not
being less than the Adequate Distance required in regard
to condition for granting of Line Clear.

(b) Block Stations where Track Circuits or other Appliances

have been provided to prove whether the connected non-
isolated lines are clear or occupied and the Signalling is
such that a distinctive Aspect is given to the Driver of a run
through Train, restricting the speed to 50 Kmph when a
connected line is occupied.

(c) Catch & Slip Sidings and Sidings provided for Isolation
purposes only.

Isolation, in simple terms, means de-linking of two lines.

3.13.1 Requirements of Isolation for different Standards of

Interlockings:

(i) Standard-I Interlocking (Speed up to 50 Kmph): Provision of

Isolation for Main line is not mandatory.
 (ii) Standard-II, III & IV Interlockings (Speeds up to 100 Kmph, 130
Kmph & 160 Kmph respectively): Line on which the Train is to run
through should be isolated from all other lines by setting of Points
or other approved means & Interlocking has to be such as to
maintain this condition during the passage of Train.

3.13.2 Means of Isolation:

Indian Railways’ Signal Engineering Manual provides for following means of Isolation:

(i) The provision of short Dead end Sidings (this Siding should not
be long enough to permit of vehicles being stabled thereon).

(ii) Sidings with Trap Points: Trap Point isolates the Siding from
running line.

(iii) Sand Hump of approved Design.

(iv) Trap Points: In order to maintain Safety for through running, Trap
Points are not inserted in Main line or Through line. The exceptions to this
rule, which may be adopted after obtaining special Sanction of the CRS
are:

(a) Where other means cannot be adopted, to permit

simultaneous reception of Trains on Single line sections.

(b) To avoid a Train being brought to a stand at a Stop Signal

on a rising gradient with the possibility of the Train being
unable to restart.

(c) To trap vehicles running away from a Station. Slip Sidings

are provided for this purpose to prevent vehicles escaping
from the Station and trying to enter into the next Block
section (On Indian Railways for all Gauges the maximum
gradient permitted is 1:400, whereas 1:1200 is usually
allowed within the station yard). Slip Siding Points must be
interlocked with the Block Instruments.

(d) To avoid a Train entering from Block Section to the Station

due to heavy falling gradient. Catch Sidings are provided
for this purpose to trap the vehicles coming uncontrolled
from the Block section and trying to enter into the Station
Yard. Catch Siding Points must be interlocked with the
Block Instruments.

(Refer to Figure 3.19 & 3.20).

At the Interlocked Layouts, the means of Isolation are interlocked with the related
Signals.

Earlier Scotch Blocks and Haye's Derails were used for providing Isolation but these are
now obsolete. Scotch Blocks and Haye’s Derails are derailing devices for a vehicle,
should it roll down to infringe the running line. A Scotch Block is a triangular piece of
metal placed on the running line and padlocked in that position. For movements over
the Block it is lowered in the off side of the rail. The action of the Scotch Block is not
positive. Cases of vehicles climbing the Block and returning to the rail have indeed
occurred.

3.14 CENTRALISED VS DISTRIBUTED OPERATION OF

SIGNALLING GEARS AT A STATION:

Mechanism of Operation of Signaling Gears at a Station – whether Centralised or De-

centralised (or Distributed) essentially depends upon the type of Signalling Technology
employed. In Mechanical Signalling Technology, it is just not possible to achieve
Centralized Operation in a major Yard because the Orthodox or Double Wire Mode of
Mechanical Operation simply does not permit Operation of Signals and Points beyond a
certain workable range. For Example, distance at which Points may be worked by
Rodding through Levers with 150 mm (6 inch) Stroke, can not exceed 320 m for Singe
ended Point and 180 m for Double ended Point. With Levers providing Stroke of 200 mm
(8 inch), the above distances can be increased to 460 m & 275 m respectively. With
Single Wire Transmission, the Signals can be worked from a distance of 950 m. If
Double Wire Technology is used, a range of Operation of 500 m can be achieved for the
Points & Locks with 500 mm (20 inch) Stroke Drum and 730 m with 610 mm (24 inch)
Stroke Drum. Signals without Detectors can be worked up to 1200 m and up to 600 m
for a Single or Coupled Lever Transmission. In Electrical Operation, with 110 V DC Point
Machines Operating in Electrified Areas, Points can be directly fed up to a distance of
1.1 Kms. On the other hand for Electrical Operation of Points with 3-Ph AC Point
Machines, there is no such prescribed physical Limit. Operation is possible up to a
distance permitted by the Voltage drop in the connecting Cable.

Centralised Operation of Signalling Gears at a Station is, thus, only possible in Electrical
Signalling Technology, though Double Wire Mechanical Signalling Technology also
permits Centralised Operation but to a limited extent only at a wayside Station.

Centralised Operation has its own distinct advantages in as much as that there are no
conflicts arising out of co-ordination issues amongst multiple Operational Agencies and
more importantly, the Centralised Agency has the full solo Control of the Yard with the
result that Planning of Movements in the Yard can be done meticulously and judiciously.

In Distributed Operation, multiple Agencies get engaged in Operational Activities. Each

Agency takes care of portion of the Yard falling in its jurisdiction. The co-ordination
amongst these multiple Agencies is achieved by ‘Slotting Mechanism’. For Reception
of a Train at a wayside Station, for example, three Agencies get engaged – Cabin on the
Reception side of the Yard to set & lock the Route, Cabin on the other side of the Yard
to set & Lock the Overlap required for Reception and the Station Master at the Centre for
overall Control on Reception & Departure Signals as also for nomination of the
Reception Lines.

Operational Time of the Signalling Gears, thus, is significantly more in case of De-
centralised / Distributed Operation in comparison to Centralised Operation. This puts an
avoidable limitation in the achievable Line Capacity on the Section.

A Signalling Scheme catering for Centralised Operation of Signaling Gears through

Panel Interlocking / Route Relay Interlocking / Solid State Interlocking, with the provision
of Block Proving through continuous Track circuits or Digital Axle Counters would
significantly reduce the time required for Signalling Operations besides providing
numerous Safety Features in the System which can, to a fair extent, eliminate the
dependence of human elements in vital checks and verifications.

(i) Centralised Operation should obviate the necessity of transferring /

receiving Slots / Controls from different Operating Agencies prevalent in a
Distributed Operating System. Also the Centralised Operation has a ‘Total
View’ of the Operations in the assigned area of Control, in front of the
Operator. Decision Making is, therefore, faster and most appropriate to
the demanding situations.

(ii) Last Vehicle Checking is a very important activity on the part of Station
Operating Staff to ascertain that Last Vehicle has arrived ‘Complete’ at
the Arrival Station and the Block section in rear is clear. BPAC does it
automatically, saving time and dependence on human element. Location
of Transducers on the Track ensures that Fouling Marks are not infringed.
Figure 3.21 shows schematic of Block Proving through Axle Counters
(BPAC). Block working by Axle Counters, where installed, has resulted in
about 10% to 12% increase in Line Capacity.

3.15 INTER-SLOTTING BETWEEN CABINS:

Slotting, in Signalling Terminology, is an arrangement in which taking ‘OFF’ of a Signal

requires the releasing of a Control by a Source or Sources other than its Operating
Source. Signals, for convenience, are worked from the nearest Source but when they
control lines, which extend into adjoining territories, they are Slotted (also called
Controlled) from Cabins / Locations located in all such contiguous territories. Slotting
Control includes the following:

(i) A Slotted Signal cannot be taken ‘OFF’, unless the Controls from all
remote locations have been operated.

(ii) It should be possible for any one of the Controlling Agencies to replace
the Signal to its most restrictive Aspect, should such a need arise in
emergent situations. This would also mean that other Signals, which are
dependent upon or released by the Slotted Signal should also be similarly
slotted so that Aspects displayed by all these Signals are in
correspondence.

(iii) For One Slot Clearance, only One Clearance of the Slotted Signal should
be possible. This is in line with the principle of One Operation and One
Clearance of a Signal. Second Clearance of the Slotted Signal should
require granting of Slot from all the Controlling Agencies, a second time.

As could be seen from the above, the purpose of ‘Slotting’ is one to ensure that the
Points located on the line controlled by the Signal including the Overlap are set correctly,
Facing Points have been locked, LC Gate - if any in the Signal run / Overlap closed &
locked against road traffic and the line is clear before the Signal is taken ‘OFF’, and
these conditions are maintained until the movement is completed. In other words, no
other conflicting or fouling movement is allowed to take place and the Points are not
altered and LC Gate – if any is not opened until the intended Train Movement gets
completed.
Figure 3.22 illustrates the principles of Slotting. Home Signals ‘3’, ‘4’ & ‘5’ are worked
from the Cabin ‘A’ for convenience but they lead into the territory, controlled by the
Station Master and Cabin ‘B’. The Block Instruments are located in Station Master’s
Office and he has an overall Control over Movements and is responsible for Reception
line Nominations. Before the Home Signal can be taken ‘OFF’ for any of the reception
lines, the line must be clear up to the far end Trailing Points and Signal Overlap (SO) in
advance of such Points. Home Signals are to be, therefore, Controlled / Slotted from
Cabin ‘B’ as well as by the Station Master.

The Outer Signal ‘2’ is released by any of the Home Signal and therefore, it should also
be slotted so that it returns to ‘ON’ position as soon as the Home is returned to ’ON’
position by Cabin ‘B’ or Station Master in an event of emergency. The Warner Signal ‘1’
should similarly be slotted from ‘B’ Cabin as well as the Station Master so that any one
of them can replace it and it can be ensured that:

(i) Main line is clear.

(ii) All Points are set and locked for the Main line Movement.
(iii) Permission to Approach from the next Station in advance has been
received.
(iv) All Main Line Signals have been taken ‘OFF’, and
(v) All the above conditions are maintained until the Train Movement is
completed.

The Advanced Starter Signal ‘18’, leading to Block section, is interlocked with Block
Instrument and slotted by the Station Master.

Following Table provides the details of Slotted Signals in the layout of Figure 3.22 and
their controlling Agencies.

Signal Operating Controlling / Slotting Agencies

Agency
Station ‘B’ Cabin Any Other
Master
Home ‘3’ ‘A’ Cabin Slide ‘2’ Lever ‘2’ -

Home ‘4’ ‘A’ Cabin Slide ‘3’ Lever ‘3’ -

Home ‘5’ ‘A’ Cabin Slide ‘4’ Lever ‘4’ -

Outer ‘2’ ‘A’ Cabin - - Home Signal ‘3’

or ‘4’ or ‘5’ of ‘A’
Cabin.

Warner ‘A’ Cabin Slide ‘1’ Lever ‘1’ Outer ‘2’ and
‘1’ Main Home ‘3’ of
‘A’ Cabin.

Advanced ‘A’ Cabin Slide ‘5’ - Interlocked with

Starter block
‘18’ Instruments.
3.16 PANEL & ROUTE RELAY INTERLOCKING SYSTEMS:

Interlocking Systems ensure that Signal for any movement is cleared only when the
Route including Overlap is properly set & locked and is free of all conflicting movements.
Also, once the Train has accepted the Signal, it should not be possible to alter the Route
till the Train has traversed over it.

In Relay Interlocking Systems, Relays are employed to achieve above functions. In

such Systems, the layout of the Yard is depicted on a Panel called the ‘Control Panel’.
Yard is illuminated, indicating the position of Points, the Route Set, Aspects of the
Signals, condition of the Track circuits - Occupied or Clear and position of running Train,
when a move is initiated.

In some versions of Route Relay Interlocking, particularly at Major Stations, the

illuminated Yard Diagram - popularly known as Indication Panel, is separate from the
Control Panel.

Two distinct types of Relay Interlocking Systems have been used on Indian Railways –
the ‘One Control Switch (OCS)’ System and the ‘Entrance – Exit (NX)’ System.

Systems employed in early Years were known as ‘One Control Switch (OCS)’ Systems
wherein to set a Route, all that required was to turn the relevant Thumb Switch to
Reverse position. When a Route Thumb Switch is turned, the Route it sets up is
indicated on the Indication Panel by the lighting of a Row of small White lights. When a
Train enters the Track circuit past the Signal, the White Indications turn to Red on the
corresponding portion of the track on the Panel. The Route lamps get extinguished only
when the Operator restores the Thumb Switch back to Normal. Safety in Train
Operation is ensured by preventing a Signal being cleared on an occupied line with the
help of Track circuits and by electrically holding the Points in their last operated position
till the Train had passed over them.

OCS type of Relay Interlocking System is now Obsolete.

System presently in use is of the ‘Entrance – Exit (NX)’ type. The Term ‘NX’ is
descriptive of the method of the Operation. At the Entrance to each Route on the Control
Panel is fitted a Push Button / Switch known as Entrance Button / Switch and at the Exit,
another Push Button, called the Exit Button. A Route is cleared by simultaneously
operating the relevant Entrance & Exit Switches / Buttons. Turning of Switch & pressing
of Button or depressing of two Buttons, automatically sets & locks all the Points and if all
the needed conditions are fulfilled, clears the relevant Signal. In present day NX
Systems, Push Buttons are only employed both at Entrance and at Exit of the Route. In
Push Button Operation, unlike in the earlier OCS System and in the presently used NX
System with Switches at the Entrance, the Route gets released automatically after the
passage of the Train. Hence no Cancellation or Release Operation need be performed
by the Operator after the passage of a Train.

The Panel Diagram in the today’s NX Systems consists of Domino type sub-Assemblies.
The Domino Panel employs rectangular Panel sections having different Track
Configurations. Any alterations on the Panel due to alteration in the Yard layout can,
thus, be accomplished by replacing the relevant Panel sections (Dominos) only.

NX Systems, from the Operational point of view can be categorized into two types.
Earlier NX Systems, some of which are still in use on Indian Railways, fall in the
Category of ‘Non-Route Setting’ type. Today’s NX Systems are, however, of the second
variant, called ‘Route Setting’ type.

In Non-Route Setting type System, the Route is set by operating individually the Points
in the Route, Overlap and in Isolation. Points are either operated by turning the Point
Switches either to ‘N’ or ‘R’ or by pressing Individual Point Button (WN) located near the
Point and a common Point Operation Button, as required. When the Points are correctly
set and locked in the required position, the concerned Indication ‘N’ or ‘R’ will appear on
the Panel. Process checks all the conditions necessary for Operation of the Point that
the Point is Free from any Route, Crank handle is ‘IN’ and the Point Track (s) is (are)
clear. After ensuring that all the required Points are correctly set and locked and
Indications are available, concerned Signal Switch is turned. This energises the Signal
Knob Relay ‘RR’. ‘UCR’, the Route Checking Relay, now picks up, the ‘ASR’ (Approach
Lock Stick Relay) drops and the Route is locked. Signal assumes ‘OFF’ Aspect, if all
other conditions required are fulfilled. ‘OFF’ Indication, thereafter appears on the Control
Panel.

In Route Setting type Panel Interlocking System, Setting of the Route, Locking and
Clearance of Signal are done simultaneously at the same time as against in Non-Route
Setting type System, wherein the activities of Setting of the Route and Clearance of a
Signal are done separately.

In Route Setting type System, Entry Signal Switch / Button and Exit Route Button are
operated / pressed simultaneously. Route Selection / Initiation / Setting Relay (LR) picks
up. LR will operate all the Points in the Route / Overlap and Isolation, to the required
positions automatically and once all the Points are correctly set and locked and
concerned Point Indication NWKR / RWKR are available, Route Checking Relay (UCR)
will pick up and ASR along with Overlap Controlling Relay OVSR will drop. TRSRs /
TLSRs (Track Stick Relays for Left to Right and Right to Left movements) shall also
drop, if the Installation is equipped with the facility of Sectional Route Release (SRR).
With the dropping of ASR, OVSR and TRSRs / TLSRs, the Route gets locked and the
Signal is taken ‘OFF’ immediately provided all other conditions for clearing the Signal are
available.

When a Route is Set & Locked, the Route is illuminated by White Strip lights in the Track
circuit configuration throughout the Route. The Indication turns to Red, when the Track is
occupied. The White Route light Indication remains lit as long as the Route is locked and
disappears only after the Route is released.

Non-Route Setting type Systems were loosely termed as ‘Panel Interlocking’ and were
restricted to smaller Yards. In the present scenario, both Panel Interlocking Systems &
Route Relay Interlocking Systems use NX Scheme, employing Push Buttons at Entrance
& Exit of the Route and Route Initiating, Setting, Locking and Releasing circuits are alike
in both the Systems.

From the Technology used point of view, the NX Systems can be divided again in two
Categories, one following the Continental practice (Siemens Type) (using Metal to Metal
Contact Relays) and the other the Anglo-American practice (using Metal to Carbon
Contact Relays). The former System extensively employs Interlocked Relays and uses
Sequence proving for all Relays including the Track Relays. The latter System mostly
employs Neutral Relays. The proper functioning of almost all the Relays in the
Interlocking circuit is proved in this System too.
Comparison between the two Systems viz. Metal-to-Carbon Contact Relay & Metal-to-
Metal Contact Relay circuit is given in the following Table:

Item of Metal-to-Carbon Contact Metal-to-Metal Contact Relay

Comparison Relay Circuits Circuits
Circuit Design Design of Signalling Circuits is Metal-to-Metal Contacts can
Simpler, as Sequence Proving stick / fuse / weld and as such
of Relays picking up and the Circuit Design essentially
dropping away in same has to take care of this
operation is not always problem. In such circuits,
required due to the therefore, a Relay picked up in
constructional feature of the an operation is proved down /
Relays used. Metal-to-Carbon de-energised in the same
Contacts Relays do not stick / operation. Circuit Design, as
fuse / weld. such, becomes a little
complicated.

Not Proving of Sequence can Sequence Proving is a healthy

sometimes be problematic due Concept and obviates the
to mechanical failure of the unsafe side failures of a Relay
Armature of a Relay sticking getting stuck in energised
up in the energised condition position. Metal-to-Metal
of the Relay, though such Contact Relay circuits are,
chances are far remote. therefore, superior to those
with Metal-to-Carbon Contact
Relays.

System Cost is lesser as System is costlier as

compared to Metal-to-Metal compared to Metal-to-Carbon
Contact Relays circuits, as Contact Relays circuits, due to
Sequence Proving is not need of additional Relays
always required in such required for Sequence
Circuits. Proving.

Design of Control Not of Domino Pattern always. Domino type Sub-assemblies

Panel are used as a Standard.

Operating Uses either Knob at the Self Restoring type Push

Buttons / Knobs on Entrance & Push Button at the Buttons are only used at
the Control Panel Exit or Push Buttons both at Entrance & Exit of the Route.
the Entrance & Exit of the
Route. Due to usage of Self-restoring
type Push Buttons only at the
With Knobs at the Entrance, Entrance of the Route, the
the Operator is required to Operator is not required to
normalise the Knob for release the Route by
releasing the Route. normalizing the Signal Switch
after passage of the Train.

3.16.1 Advantages of Relay Interlocking over Mechanical

Interlocking achieved through Lever Frames:
 (i) Centralised Operation obviates the necessity of transferring /
receiving Slots / Controls from different Operating Agencies
prevalent in a Distributed Operational System. Also the
Centralised Operation has a ‘Total View’ of the Operations in his
area of Control, in front of him. Decision Making is, therefore,
faster and most appropriate to the demanding situation.

(ii) Route Setting & Clearance of Signals is done electrically, which

avoid delays to the Train running and thus enable an increased
traffic density to be handled at a Station. Panel operation of
Signals & Points significantly reduces the Operating Time for
Operation of Signalling gears.

(iii) Last Vehicle Checking is a very important activity on the part of

Station Operating Staff to ascertain that Last Vehicle (LV) has
arrived complete at the arrival Station and the Block section in
rear is clear. Block Proving through Axle Counters (BPAC) does it
automatically, saving time and dependence on human element.
Location of Transducers on the Track can ensure that Fouling
Marks are not infringed. Block working by Axle Counters, where
installed, has resulted in about 10% to 12% increase in Line
Capacity.

(iv) Safety Features built in the System, to a fair extent, eliminate the
dependence of human elements in vital Checks and Verifications.

(iv) Maintenance and up-keep of the Installation is easy & smooth.

There is also no possibility of Outside Interference and Miscreant
Activity, which is predominantly present in a Mechanically
Interlocked system.

3.17 CENTRALISED TRAFFIC CONTROL (CTC) SYSTEM:

Centralised Traffic Control (CTC) System refers to Remote Control of Interlockings of

wayside Stations on a Section from a Central Control Centre (CCC). Commands to
operate the Signalling Gears at a Station for clearing a Reception or Departure Signal
are initiated at the Central Control and Status of the Command Execution i.e. the Status
of Points, the Routes Set and the Signals Cleared & Aspects thereof is communicated,
in Real Time, to the Control Centre for the Display on the big Wall Indication Panel
located in front of the Controllers. The Indication Panel covers the entire Section under
control from the said Control Centre (CCC) and displays a Schematic Layout of the
whole line including the Stations with all Tracks & Status of the Signalling Gears – the
Points, Track circuits & Signals and the Routes Set etc. In a more advanced version of
CTC, called Train Management System (TMS), even the Train Numbers can be
displayed in small fields on the Track Lines according to the current positions of the
Trains on the Section. The Information of Train Numbers is dynamically shifted from one
field to the next according to the current Train Movements.

Under CTC working, Normal Operation of Station Interlockings is through the Remote
Control from the CCC. Local Operation from Local Interlockings is resorted to only in
Emergency cases or for undertaking Shunting Movements. Certain Large Stations can,
however, be permanently controlled through Local Operation, depending on the Local
conditions. Train Numbers of different Train Categories can also be displayed in different
ways, for example, by using different Digit Colours.

Since CTC includes Centralised Control, Panel Interlockings (Relay Interlocking / Solid
State Interlocking) at wayside Stations, elimination of necessity to verify the Complete
Arrival of Trains (Continuous Track circuiting or Block Proving through Axle Counters),
Tokenless Block working (on Single Lines) and / or Automatic Block Signalling, the
potentialities of CTC in increasing the Line Capacity are significant. Main Benefits of
providing CTC are:

(i) Increase in Line Capacity: The provision of Centralised Traffic

Control on congested Single line system increases their Line Capacity to
the extent of about 40% - 50%. Increase in Capacity is due to the
following features:

(a) Reduced Crossing Time: Time taken for picking

up of Token, bringing it upto the Block Operating location,
extracting a new Token and handing over the same to the
Driver accounts for a total time of 10 to 15 minutes. With
CTC, it is possible to take off the Signals for the first Train
while the second Train is still moving and has not
completely stopped at the Station. Not only this, driver of
the first Train remains aware of the time as to when the
Signal for his Train shall be taken ‘OFF’ after arrival of a
Train from opposite direction and, therefore, he starts
practically within half a minute or even earlier after the
Signal has been cleared for his departure.

(b) Quick Starting of the Goods Trains: Normally

when a Goods Train is brought to a stop, the Driver does
not know when he is likely to be allowed to proceed ahead,
particularly when it happens to be a time for Passenger
Trains. He, therefore, takes it easy and does not worry
about and when the Signal is taken ‘OFF’, either he is not
in his Cab at all or he is not ready to start due to one
reason or other. This accounts for further delay in
departure of the Train. In case of CTC, Driver has to be
always alert as he knows that his Train will be kept on
moving and may be stopped for overtaking or crossing
purposes for a few minutes only. Further if he delays
starting of his Train, he knows that it is going to be
detected and got recorded on Automatic Train Graph
Recorder. He is, thus, always conscious that in case of
failure on his part in not starting the Train in time, he is
likely to be questioned. Fear of this detection and
questioning ensures that the Driver will start his Train as
soon as the Signal is taken ‘OFF’.

(c) Overtaking of Trains: Provision of Automatic

Block Signalling enables the overtaking operation to be
over within a minimum possible period. While a Goods
Train or a slow passenger Train is approaching a Station,
there may be a faster Train in rear and as soon as the slow
 Train is berthed on one of the lines at the Station, the
faster Train is given precedence. Again after the faster
Train has cleared the next Signal, the Goods Train / slow
Passenger Train can start immediately behind it. Thus
detention to slow moving traffic i.e. the Goods Trains are
reduced considerably. This, in turn, results in better
average speed of Goods Trains and Shunting Van Trains.

(d) Proper Planning of Train Movement: CTC

System places complete information and execution of
various functions in the hands of the Operator on the
Central Console. This means that there is no delay in
ascertaining the location of Trains in the Section or in
acting to change Routes to take care of unexpected
conditions since it is unnecessary either to call and
question a remote Station Master or to transmit
Instructions nor is there any hindrance in expeditious
handling of the situations because of changed conditions.
In addition, elimination of Intermediate Operators viz. the
Cabinmen and Station Masters etc., removes the
opportunity for division of responsibilities.

(ii) Increased Safety in Train Operation: Because of the complete Track

circuiting of the Block section as well as Station Yards, the Safety of Train
Operation is increased, as it is no more possible to clear any Signal for an
occupied line. The Visibility of Colour Light Signals is also better which
further aids the Driver in better control of his Train.

(iii) Economy in the Expenditure of Staff: With the provision of CTC, it is

possible to eliminate most of the Operating Staff from the wayside
Stations. Staff will only be required at wayside Stations for the following
purposes:

(a) Commercial work.

(b) Keeping a watch on the Axle of Moving Trains (for

detecting of Hot Boxes etc.).

(c) Controlling the Shunting Operation.

With the Modular Design of the System, wherein the Maintenance Staff
will only be required to detect the particular Module at fault and then
replace it with the new one / already repaired one in a Central Repair
Shop, the strength of S&T Maintenance Staff also can be significantly
reduced.

(iv) Increased facilities in Maintenance of Permanent Way: Track

Maintenance, when we are going in for Mechanised Tamping through
Track Tie Tamping Machines is simplified with CTC. CTC Operators can
see at a glance on the Control Console what Train situation exists at any
given location. It is, thus, possible for the CTC Operator to set up Signals
and Route so that Maintenance Staff can move or work without delay and
take advantage of the maximum gap available between Trains.
CTC can be introduced at roughly 1/3 rd of the cost of Doubling of saturated Single line
sections. Whenever there is an increase in the number of Trains beyond a saturation
point on a Single line or Double line, the solution generally sought is the provision of
additional Track i.e. Doubling of Single line or providing a 3rd line in case of Double lines
and so on. Provision of additional Track means heavy initial Capital cost and increased
recurring expenditure. Provision of CTC on such congested sections at about 1/3rd of the
cost of Doubling, would result in the Sectional Capacity being increased to the extent of
40 - 50%, postponing the need of additional Track for many years to come and at the
same time yielding rich dividends on the Capital Investment. CTC Installations in many
parts of the World have shown that this Mode of Signalling makes it possible to affect
notable increase in the Line Capacity, while all other factors such as Track alignment,
gradient and form of the motive power have remained unchanged.

CTC along with Remote Supervisory Train Management System can provide complete
control of 200 to 300 Kms. of a section from a Central location and also monitor the
actual running of Trains therein to take corrective action as and when required. With
rapid pace of computerisation, powerful systems are now available to optimise the
sectional Line Capacity. It can provide for proper precedence to slow moving Trains with
faster Trains suffering only a minimum detention. Automatic Block Signalling, being an
essential ingredient of CTC, the Block Operating Time is completely eliminated. The
System can generate Line Capacity to much more than 100 Trains each way from the
present Capacity of 50 to 60 Trains.

CTC can be introduced in following phases:

(i) Provision of Tokenless Block working by using continuous

Track circuits or Electronic Axle Counters.

(ii) Provision of Panel Interlockings at wayside Stations on the

section in question.

(iii) Provision of Automatic Block Working on the section.

(iv) Provision of Remote Supervisory Train Management System

with Train Describers.

(v) Provision of equipment for conducting the Operations at

various wayside Stations from the Central Control Centre Office.

3.18 FLYING CROSSINGS & PRECEDENCES:

With the introduction of CTC on Single lines, the Line capacity can be increased to the
extent of 40 – 50%. If a further increase in the Capacity is required, it could be done by
providing Flying Crossings, thereby enabling two opposite Trains to cross without either
of them stopping either at the foot of the Signal or at the Station. This is a concept, which
can be used prior to undertaking partial or full Doubling of the section. It is, however,
necessary to foresee with certainty as to at which Station (s), the extended loops for
providing such crossings would produce the best results. The lengths of loops required
for Flying Crossings will depend upon the length and speed of the Trains.

On Double line sections, extended loops can be provided at select Stations to cater for
Flying Precedences on the lines similar to Flying Crossings. A sample study done on
one of the busiest section of Indian Railways, has brought out that in comparison to the
Third line Project on the same section, the proposal of extended loops with Automatic
Signalling, CTC and Remote Supervisory Train Management System would cost 31%
less and meet the requirements. Concept of Flying Precedence is shown in Figure 3.23.

3.19 INTERMEDIATE BLOCK SIGNALLING (IBS):

Intermediate Block Signalling (IBS) is an Arrangement of Signalling on Double Line

Section which is a kind of Substitute for a Class 'C' Station and is used for the purpose
of splitting of a long Block Section into two portions. IBS does not require manning by
Operating Staff, which is a necessity for a Class ‘C’ Station. Station Building is not
required and the recurring cost of Operating Staff is also saved.

IB Signalling can be provided either with continuous Track circuiting between the Last
Stop Signal (LSS) of the Station in rear up to the Overlap of the IB Signal or with Axle
Counters with the Entrance and Exit Points located at the LSS and at the Overlap of IB
Signal respectively. The IB Signal is controlled by the Station Master of the Block
Station in rear. On a number of Railways, IB Signalling Installations are working
satisfactorily and are giving good improvement in the running especially of Goods Trains
since IBS permits two Trains to be present in the Block Section at a time as against one
Train with the Absolute Block System and this considerably minimises detention for the
second Train when a slow earlier Train has already entered the Block Section.

For each Direction, one IB Signal at the place where the Section is divided into two and
a Distant Signal at minimum 1 Km. from IB Signal is provided. A typical Arrangement of
IB Signalling is shown in Figure 3.24.

Figure 3.24 shows the IB Signalling with Axle Counters. It is also possible to have IBS
with two Track circuits, one from LSS to IB Signal and the other from IB Signal up to the
Overlap of 400 m. To the extent possible, UP IB Signal and DN IB Signal should be kept
close so that both can be controlled from the same IB Equipment Room located close to
the Signals. This Room will accommodate the Relays controlling the IB Signals and IB
Distant Signals, Lamp Proving Relays, Track Proving Relays etc., and the AC Power
Supply for Signals as well as 12 V or 24 V DC Power Supply. While locating IB Signal,
it should preferably be located in such a way that the travel time for the Train will be
equal for both the halves of Section so that optimum Headway can be maintained
between the Trains at maximum possible speed. However, considerations like Gradient
at the Approach of IB signal and existence of Neutral Section, Visibility of Signals etc.,
are also to be considered while locating the IB Signal. If required, UP and DN IB Signals
may be staggered to suit the local conditions.

General Rules require that IB Signal shall not be taken ‘OFF’ for a Train unless Line
Clear has been obtained from the Block Station in advance.

3.19.1 Sequence of Operation for Train Movements with IBS:

When there is no Train in the Section, the LSS of the Station 'A' (Sending Station) which
is controlled by the Evaluator of the Axle Counters for the DN Line can be cleared if the
Axle Counter is showing Section Clear. In case of End Cabin working, the Clearance of
LSS by Switchman will also require the release of Control from the ASM. The DN IB
Signal is controlled by a Lever in the Cabin of Station 'A' (Sending Station) and also by
the Line Clear by Station 'B' (Receiving Station).
When the Train goes beyond IB Signal and Clears the Overlap which is 400 m in
advance of IB Signal the Route covered by the LSS Signal of Station 'A' (Sending
Station) becomes free once again either due to the two Track circuits showing Clear or
due to Axle Counter showing Section Clear once again. The LSS of Station 'A' (Sending
Station) can now be cleared to admit a Second Train into the IBS Section.

As soon as the first Train occupies the Track circuit in advance of IB Signal, the IB
Signal is replaced to ‘ON’. In case of IB Signal with Axle Counter, the IB Signal can be
replaced to ON by the Trolley Suppression Track circuit or a Track circuit specifically
provided for the purpose.

Similar shall be the working on UP Line.

As soon as the First Train goes beyond IB Signal and clears the replacing Track circuit,
‘Train Entering Section (TES)’ Signal is to be given by Cabin of Station 'A' (Sending
Station) to the Cabin of Station 'B' (Receiving Station) and the Handle of the Block
Instrument to be turned to TOL at the Receiving Station. When the Train reaches the
Bock Clearance Point in advance of the Home Signal at Station 'B' (Receiving Station)
and on getting Train Arrival Buzzer, the Block Handle will be turned to Line Closed by
Station 'B' (Receiving Station). After exchanging necessary Information on Telephone a
fresh Line Clear is to be granted by Station 'B' to the Second Train which is now
travelling between the LSS and IB Signal and when the Station ‘B’ gives the Second
Line Clear by turning the Handle of Block Instrument to Line Clear, the IB Signal can be
cleared by Cabin of Station 'A' (Sending Station) for the Second Train which is now
approaching the IB Signal and the Sequence of Events will be followed and when the
Second Train clears the IB Overlap point (400 m in advance of the IB Signal), the next
Train waiting at Station 'A' (Sending Station) can be dispatched by clearing the LSS.
Thus if the Axle Counter Controlling the IB Signal and the Double Line Block Instrument
which controls the second half beyond the IB Signal are both working properly, it would
permit one Train to be between LSS and IB Signal and the second Train between the IB
Overlap Point and the Home Signal of the Station 'B' (Receiving Station).

The Circuitry of IBS is so designed that only two Trains can be there in the entire IB
System for each Direction and the third Train cannot enter on Signals when the System
is already having two Trains in the same Direction.

3.19.2 Indications:

At the Cabin of Sending Station:

(i) Indication to show that the Axle Counter Portion is Occupied or Cleared.

(ii) Indication when the Station Master has given Slot for LSS.

(iii) Indication (Audio Visual) whenever the IB Signal is passed at Danger by a

Train, with the facility of silencing the Buzzer on Acknowledgement.
Visual Indication should, however, remain till such time the IB Signal is
put Right.

(iv) Indication when the Train occupies LSS Track circuit. This is the reminder
to the Switchman to replace the LSS Lever to Normal.

(v) Indication when the Train occupies the IB Signal replacement Track
 circuit. This is a reminder to the Switchman to put back the IB Signal
Lever to Normal.

(vi) Indication (Audio Visual) when the IB Signal goes Blank, with the facility
of silencing the Buzzer on Acknowledgement. Indication should, however,
remain till such time the IB Signal is put Right.

3.19.3 Procedure to be followed by Drivers in case of IB Signal at Danger

or Blank:

(i) Whenever the Driver comes across an IB Signal at Danger or

Blank, he shall stop his Train in rear of the Signal, advise the
Guard of the Fact by sounding one long continuous Whistle at
distinct Intervals and contact the SM / Switchman of the Station or
Cabin in rear as the case may be (Station 'A'), on Telephone if
provided on the IB Signal Post.

(ii) Station Master shall tell the Driver the reasons for the Signal
Failure and after ascertaining that the said Signal is defective,
Authorise the Driver to pass the Intermediate Block Stop Signal, if
Defective as prescribed by Special Instructions. Special
Instructions may include Exchanging of Private Numbers.

(iii) In case the Telephone is Defective or Communication is not

possible with the Station 'A' (Sending Station), the Driver shall wait
for 5 minutes at the Signal and even after waiting for 5 minutes the
Signal is not taken ‘OFF’, he shall advise the Guard of this Fact by
sounding one long Whistle which may be repeated as necessary,
Exchange All-right Signals with him, and then pass it at ‘ON’ and
proceed Cautiously at a Speed not exceeding 15 Kmph and be
prepared to Stop short of any obstruction. In case the Driver is not
in a position of having a good view of the Line ahead due to Fog &
Night conditions, etc., the Speed shall be reduced to 8 Kmph.
Driver shall proceed at restricted Speed and be extremely vigilant
till he reaches the Stop Signal of Station 'B'. Driver shall act on the
Aspect of Stop Signal of Station ‘B’ only after reaching at the
Signal. On his arrival at Station 'B' (Receiving Station), the Driver
shall report the Failure to the Station Master of the Block Station
‘B’.

(iv) The Station Master of the Block Station working the Intermediate
Block Stop Signal (Station 'A') on becoming aware that such a
Signal is Defective shall, before dispatching a Train, treat the
entire Section up to the Block Station immediately ahead of the
Intermediate Block Post as one Block Section and issue a written
Authority to the Driver to pass the Defective Intermediate Block
Stop Signal at ‘ON’ without stopping at the Signal, in accordance
with the Procedure prescribed by Special Instructions.

3.20 SIGNALLING ARRANGEMENTS FOR INTERMEDIATE SIDING:

Sidings taking off from Running lines and located between Block Stations are known as
Intermediate or Outlying Sidings. These Sidings are either used for Departmental
purposes or to serve an Industry located too far away from a Block Station, when it is
known as an Assisted Siding.

An Outlying Siding Location is not, usually manned for reasons of economy. The
responsibility for movements into and out of the Siding and for ensuring that the Running
line or lines are Clear after such Operations are completed, as also for correct re-setting
of the Points to the Normal Position (i.e. for the Running lines) is placed on the Guard of
the Train entering or coming out of the Siding. Use of Track circuits to prove that
Running lines are Clear should be a good Option but not usually adopted for reasons of
economy and problems in Maintenance. The Points of Outlying Siding whether Facing or
Trailing, whether on Double line or Single line for reasons stated above, are, therefore,
not permitted to remain un-interlocked so as to provide adequate Protection to Running
Movements.

3.20.1 Minimum Equipment for the Intermediate Siding Point:

The minimum equipment prescribed for the Points is as for Standard-I without Detection:

(i) Facing Points, Single and Double Line:

(a) A Gauge Tie Plate, where Steel Sleepers are not provided.
(b) A Facing Point Lock (or equivalent Mechanism), the
Plunger of which shall lock each Switch independently.
(c) The Control of the Points shall be by means of a Key or
other suitable Device, which shall secure the Bolting
Mechanism of the Point in the Plunged or Locked Position
when the Points are Set and Locked for the Running line.
(d) The Means for Control of the Points shall be Interlocked
with the Block System in Force.
(e) Isolation: In view of the requirements that all Passenger
Running lines must be isolated from Goods and Stabling
lines, Isolation must be provided and the Isolating Traps
locked for all the movements on the Running lines at the
Outlying Siding. A Trap Indicator has to be provided but it
is not lit during night hours.
(f) Where Siding Points are provided only with the minimum
Equipment, as detailed above, an appropriate Speed Limit
over the Facing Points shall be imposed for all Trains
passing over such Points in the Facing Direction only and
an ‘S’ Marker at the Points and a Speed Indicator at not
less than 30 m from the Facing Points shall be provided,
neither of which need be lighted. In addition, a Caution and
Termination Indicator shall also be provided as for usual
Speed Restrictions.
(g) Where the Sanctioned Speed of the Section does not
exceed 50 Kmph, the ‘S’ Marker at the Points and the
Indicators need not be provided.

(ii) Trailing points, Double line:

(a) A Gauge Tie Plate where Steel Sleepers, are not provided.
 (b) A suitable type of Key Lock (or equivalent Mechanism), the
Key of which can only be extracted when the Points are
Set and Locked for the Running line.
(c) The Points shall be controlled through the Block System in
Force.
(d) An `S' Marker at the Points, which need not be lighted.
(e) No Speed Restriction is imposed as Points are Trailing.
(f) Isolation: In view of the requirements that all Passenger
Running lines must be isolated from Goods and Stabling
lines, Isolation must be provided and the Isolating Traps
locked for all the movements on the Running lines at the
Outlying Siding. A Trap Indicator has to be provided but it
is not lit during night hours.

Schematic Arrangements are shown in Figures 3.25 & 3.26.

3.20.2 Equipment for the Intermediate Siding Point for Speeds over
50 Kmph:

If the Speed Restriction of 50 Kmph over Facing Points is to be avoided, the Point
Equipment should conform to Standard-III Requirements and a Stop Signal adjacent to
the Points Detecting each Switch and Facing Point Lock or equivalent Mechanism
should be provided. A Warner or Warning Board should also be provided at minimum
BD in rear of the Stop Signal in the case of 2-Aspect Signalling and a Distant in the case
of MA Signalling.

Schematic Arrangements are shown in Figure 3.27.

3.20.3 Sidings Located on Busy Sections:

In the case of Sidings located on Busy Sections, in which Shunting has to be performed,
Means for Clearing the Section after a Train has been berthed in the Siding, are
provided.

On the Double line as the Lock & Block Instruments cannot be restored to the Normal
Position until and unless the Train has actuated a Track circuit located near the Home
Signal of the Receiving Station, the Section cannot be cleared with the Train berthed in
the Intermediate Siding. Interlocking may, however, be provided between the Block
Instruments and the Points so that the Points cannot be unlocked without the Block
Instruments being in the TOL Position. Should the facility of Clearing the Block Section
with a Train berthed in the Intermediate Siding be considered necessary, the
Intermediate Siding is worked as a Block Station and switched out when the use of the
Siding is not required. There are, however, Double Line Block Instruments which provide
for an Occupation Key to enable Trains to enter the Block Section in the right direction
and return to the same Station. This Occupation Key when removed from the Instrument
locks the Instruments in the Normal Position. In such cases, the Occupation Key may be
exploited for sending Trains into Intermediate Siding and clearing the Section after the
Train is berthed in the Siding. This Train can only return to the Station from which it
entered the Block, travelling in the wrong direction.

In the case of Single Line Sections, Two alternative Methods are available for clearing
the Block Section with a Train berthed in the Intermediate Siding:
(i) Based on the Use of Token / Key Interlocking Boxes:

Refer to Figure 3.28.

At station ‘X’ a Token Key Interlocking Box with a spare Token locked in it
and at the Siding, a similar Box, but with the Points Key locked in it are
provided. An HKT circuit is provided between Station ‘X’ and the Siding,
a Key is locked in the HKT at ‘X’ and, therefore, there is no Key in the
HKT at the Siding end. A Train proceeds from ‘X’ or ‘Y’ with a Token and
is stopped at the Siding and the Token is exchanged for the Points Key
on the Token Key Interlocking Box. The Points Key is used to unlock the
Points. The Train enters the Siding. The Points are then reset in Normal
and after ensuring that the Running line is Clear, the Key is transmitted to
‘X’. The Key is exchanged at ‘X’ for a Token (in the Token Exchanger at
the Station) which is used to clear the Section. When the Train has to
come out of the Siding, the Guard informs ‘X’ on the Telephone of the
fact. ‘X’ obtains a Token, exchanges it for the Key and transmits the Key
to the Siding. The Key is used for unlocking the Siding Points. The Train
enters the Running line, the Points are reset to Normal and the Key
released from the Points exchanged for the Token originally deposited in
the Token Key Interlocking Box. The Train proceeds with the Token to
either ‘X’ or ‘Y’. This System allows a Train either from ‘X’ or ‘Y’ using
the Siding and the Train from the Siding proceeding to either ‘X’ or ‘Y’.
The disadvantages are that only one Train can be dealt with in the Siding
at a time and that at least two line wires one for HKT circuit and the other
for Telephone are required between ‘X’ and the Siding.

(ii) Based on the Use of two Auxiliary Block Instruments:

Refer to Figure 3.29.

A separate Auxiliary Block Instrument circuit is established between

station ‘X’ and the Siding, the Instruments being of the same type as used
in ‘X’ and ‘Y’, but arranged to be out of Phase so that a Token cannot be
extracted from either of the two. A Token Key Interlocking Box in which
the Points Key is normally locked is provided at the Siding.

A Train from either ‘X’ or ‘Y‘ may proceed to the Siding. On arrival at the
Siding, the Token is exchanged for the Points Key. After the Train enters
the Siding and the Running line is ensured to be Clear, Points are reset
and locked in Normal and the Key exchanged for the Token. The Token
is then inserted into the Block Instrument which brings the pair of Auxiliary
Instruments in Phase and enables Station ‘X’ to extract a Token from the
Auxiliary Instrument at his Station. This Token is then used for clearing
the Section on the Block circuit ‘X’ - ‘Y’. The Auxiliary Instruments are
now again out of Phase. A similar Procedure is adopted for coming out
of the Siding. This System permits any number of Trains being berthed in
the Siding but requires an extra line wire for the extra pair of Block
Instruments.

3.21 SIGNALLING FOR CATCH & SLIP SIDINGS:

3.21.1 Catch Siding:

Catch Siding is a Siding provided with an upward incline coming off a downward track, to
stop a runaway Vehicle. Facing Points connected to the Siding and Main Line are
normally set for the Catch Siding. After the Train has come to a halt at the protecting
Signal, the Points are set for the Main Line and the Signal taken ‘OFF’. Train now can
proceed further on Main Line. Should a runaway Train reaches the Points at speed, it is
diverted on to the Catch Siding which in addition to being inclined upwards also has
loose sand on the track bed to absorb the kinetic energy of the Train and bring it to a
halt. Catch Sidings are usually provided just after the Outermost Stop Signal of the
Station or Yard.

3.21.2 Slip Siding:

Slip Siding is a Siding provided going off the Main Line away from a Station. Slip Siding
prevents runaway Trains that escape from the Station Limits or Yard Limits from
continuing on the Main Line. Normally the Points are set to divert all Trains to the Slip
Siding. This way only authorized Trains can proceed on to the Main Line. Slip Sidings
are usually provided to the rear of the Last Stop Signal of any line leading out of the
Station or Yard.

3.21.3 Mandatory provisions:

Rules for Opening a Railway requires that at any Station situated in the immediate
neighbourhood of an incline steeper than 1 in 100 falling away from the Station and 1:80
falling towards the Station, a Catch Siding in the former case and a Slip Siding in the
latter case, should be provided, in a suitable position. The take off Points to a Catch
Siding or a Slip Siding should normally be set and locked for the Siding, except where
required to be Trailed Through.

Figures 3.30 & 3.31 provide for typical Layouts for Slip & Catch Sidings. Figure 3.31
shows a Layout with both Catch & Slip Siding at the same Station.

3.21.4 Interlocking of Catch Sidings and Slip Sidings:

Indian Railways’ Signal Engineering Manual provides for following Arrangements:

(A) Reception of Trains:

(i) Either a minimum of two Stop Signals shall be provided in rear of

the Catch Siding Point or the First Stop shall be at an Adequate
Distance equal to Block Overlap from the Catch Siding Points.

(ii) A Train shall be brought to a stop at the First Stop Signal, before
the Catch Siding Points are set for the Main Line and the
Reception Signals taken 'OFF' unless the following conditions are
satisfied:

(a) The Line on which the Train is to be received is

Clear and the Train is to be received on the Main
line.
 (b) The Points leading to the Catch Siding as also all
the Points required for a Run Through Train are set
for the Main Line immediately after granting Line
Clear to the Block Station in rear.

(c) Line Clear has been obtained from the Block

Station in advance.

(d) The Gradients in the Block Section ahead are such

that the Train can be brought under control easily.

(B) Setting of Catch Siding / Slip Siding:

The take off Points of a Catch / Slip Siding shall normally be set and locked for the
Siding and Interlocking between the Points and Block Instruments shall be provided
meeting the following requirements:

(i) The Interlocking shall be such that the Key required to set the Siding
Points is released from the Instruments in the ‘Train Going to’ / ‘Train
Coming From’ position and once the Key is removed from the Block
Instrument, the Instrument gets locked in the relevant position. The
instrument can be normalised only after the Points are set for the Catch
Siding / Slip Siding and the ‘Train Going To’ / ‘Train Coming From’ Key is
released from the Points and brought back to release the Instrument.

(ii) Where a Slip Siding is located at the departure end of a Double Line
Station, the interlocking shall be such that the Points can be set towards
the Block section only when the block instrument is set to ‘Line Clear’.

(C) Audio Visual Indication:

An Audio Visual Indication shall be provided at the place of operation of Points as an aid
to the Operating Staff, indicating that the Train has been received or dispatched and that
the Points can now be reset for the Catch Siding / Slip Siding. This Indication would
continue till the Points are reset for the Catch Siding / Slip Siding.

(D) Other Safeguards in Working:

In all cases where the Interlocking Arrangements referred to above are provided
between the Catch / Slip Siding and the Block Instrument, the following safeguards shall
also, inter-alia, be provided in the Station Working Rules:

(i) Shunting on non-isolated lines shall cease once the Line Clear has been
granted;

(ii) Points shall remain set and locked for the Catch Siding until Home Signal
has to be taken 'OFF'.

(iii) Before Normalising the Block Instrument the Assistant Station Master /
Cabinman shall verify the Complete Arrival or Dispatch of the Train, even
 if he is getting an Audible Warning, which can arise due to Reception or
Dispatch of a complete Train or part thereof.

(E) Emergency Key:

An Emergency Key of the Catch / Slip Siding shall be kept in a sealed Box under the
Custody of the Station Master. This is used for operating the Catch Siding / Slip Siding
Points when either the Block Instruments have failed, or when the Train is still in the
Block Section and a Train is required to be Dispatched into the Block Section.

*******