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Signaling
9.1 General
The purpose of signaling and interlocking is primarily to control and regulate the movement of
trains safely and efficiently. Signaling includes the use and working of signals, points, block
instruments, and other allied equipment in a predetermined manner for the safe and efficient
running of trains.
9.2 Objectives of Signaling
The objectives of signaling are as follows.
(a) To regulate the movement of trains so that they run safely at maximum permissible speeds.
(b) To maintain a safe distance between trains that are running on the same line in the same
direction.
(c) To ensure the safety of two or more trains that have to cross or approach each other.
(d) To provide facilities for safe and efficient shunting.
(e) To regulate the arrival and departure of trains from the station yard.
(f) To guide the trains to run at restricted speeds during the maintenance and repair of tracks.
(g) To ensure the safety of the train when it comes in contact with road traffic at
level crossings.
9.3 Types of Signaling
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A. Audible Signals
B. Visible Signals
a. Hand signals
b. Fixed signal
i. Caution indicators
ii. Stop signals
A. Operational
Communication of message in audible or visual form
Examples Audible: Detonators Visual: Hand signals, fixed signals, etc
B. Functional
C. Locational
D. Special Signals
Meant for special purposes
Calling-on signals, repeater signals, speed indicators, etc.
9.4 Classification according to function
Signaling the driver to stop, move cautiously, proceed, or carry out shunting operations
Stop signals, shunt signals, speed indicators
9.5 Classification according to location
Reception or departure signals
Outer, home, starter, and advanced starter signals
9.6 Special Signals
Meant for special purposes
Calling-on signals, repeater signals, speed indicators, etc.
9.7 Typical layout
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9.8 Control of Movements of Train
The system adopted for controlling the movement of trains should be such that it allows the trains
to run in either direction as well as facilitates faster trains to overtake slower trains, thus ensuring
the complete safety of trains. The following systems are chiefly used for controlling the movement
of trains on Indian Railways.
Time interval system
In this system, there is a time interval between two successive trains. A train is dispatched only
after sufficient time has elapsed since the departure of the previous train. This system works fine
just as long as everything goes well with the previous train, but if there is a mishap and the previous
train is held up, the system fails, jeopardizing the safety of the trains.
Space interval system
In this system, there is a space interval between two consecutive trains. Only one train is permitted
to occupy a particular length of the track. A succeeding train is permitted to occupy the same track
length from either side only after the first train has cleared it. This system guarantees safety as
only one train is in motion at one time.
Methods of Controlling Train Movement
Based on these systems, the following methods are adopted for controlling the movement of trains
on Indian Railways.
One-engine-only system
This system permits only one train to remain in a section at one time. The movement of trains is
controlled with the help of a wooden staff or a token with suitable identification marks, which are
in the possession of the driver of the train. As the same object cannot be at two places at the same
time, the safety of trains is fully ensured. This system is possible only on short branch lines that
have limited traffic. Normally there is only one train, which works to and fro on the same section.
The system fails if it becomes necessary to dispatch more than one train in the same direction. This
system does not require a 'line clear' directive.
Following-train system
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In this system, trains follow each other after a time interval that is generally less than 15 min.
Trains scheduled after the first train can run at a maximum speed of 25 km/h. As an adequate time
interval is kept between two successive trains, safety is ensured to a limited extent. The system is
used under the following circumstances.
(a) In the case of emergencies such as the failure of block instruments and the telephone system
(b) In short double-line stretches
Pilot guard system
In such a system, one person, known as the pilot guard, accompanies a train by riding on the foot
plate of the engine (or gives a ticket personally to the guard of the train, which is authority to
proceed) and returns to the same station with another train. The pilot guard is normally identified
by his or her prescribed uniform, which is red in colour, or the badge that he or she wears and is
an authority for the train to proceed. Even in this system trains can follow each other after a fixed
time interval of not less than 15 min. The system is applicable in short single-line sections or in
the case of failure of communication between two stations.
Train staff and ticket system
This system is similar to the pilot guard system. The authority to proceed in this case is either a
wooden staff or a ticket. There is only one wooden staff for a section and the same is kept at one
of the two stations on that section. Each station has a ticket box which contains printed tickets and
is kept locked. The wooden staff is interlocked with the box in a way that it cannot be taken out so
long as the box is locked. A train can only be dispatched from the station that has the staff. In case
only one train is to leave the station, then the staff is handed over to the driver of the train. If more
than one train is to be dispatched from the same station, the preceding trains are dispatched on the
authority of the ticket while the last one is dispatched along with the staff. The time gap between
two successive trains is not less than 15 min and the speed of the trains is restricted to 25 km/h. A
similar system is followed for dispatching trains from the other station. In this system, the safety
of the trains is ensured on account of the fact that only one ticket can be issued at one time and the
driver insists on seeing the staff before accepting the ticket as his authority to proceed.
Absolute block system
This system involves dividing the entire length of the track into sections called block sections. A
block section lies between two stations that are provided with block instruments (explained later).
The block instruments of adjoining stations are connected through railway lines and a token can
be taken from the block instrument of a particular station with the consent of both the station
masters.
In the absolute block system, the departure of a train from one station to another is not permitted
until and unless the previous train has completely arrived at the next station, i.e., trains are not
permitted to enter the section between two stations at the same time. The procedure by which this
system is maintained is known as the lock and block procedure. The instruments used for this
purpose are known as block instruments.
Block instruments Each station has two block instruments; one for the station ahead and the other
for the previous station. The block instruments of two adjacent stations are electrically
interconnected. These block instruments are operated with the consent of the station masters of the
stations on either end of the block section, who are also responsible for giving the line clear
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indication. Normally a round metal ball called a 'token' is taken as the authority to proceed in a
block section. This token is contained inside the block instrument.
There following different types of block instruments are used on Indian Railways depending upon
various requirements.
Single-line token instruments
These are meant for stations with single lines. No train is authorized to enter the block section
without a token. The token can be taken out of the block instrument of the departure station only
when the station master turns the handle of the block instrument towards the end labelled 'Train
going to side'. This can be done only with the consent of the station master of the station on the
other side of the block section, who turns the handle of his or her block instrument towards the end
labelled 'Train coming from side'. It is not possible to turn the signal permitting the entry of the
train into the block section off until the handle of the block instrument has been turned towards
the 'Train going to side' label. In this situation, the handles of both these instruments get locked in
the last operated position and it is not possible to normalize both the block instruments until the
train arrives at the next station and the token has been inserted into the block instrument of that
station. This phenomenon of keeping the block instruments locked and releasing them only during
the passage of a train is the previously mentioned lock and block procedure.
Single-line tokenless block instruments There have been occasions when a train has had to be
brought to a halt because of the driver misplacing the token, causing the trains to get detained for
long periods. In order to avoid such occurrences, tokenless block instruments have been developed.
The same principle as that of the block system is followed here but without the use of a token. The
last stop signal permitting the entry of the train into the block section, which is normally the
advanced starter signal, is interlocked with the block instrument in such a way that it is not possible
to turn this signal off unless the block instrument has obtained the line clear command.
Double-line block instruments In a double-line section, traffic is unidirectional. The block
instrument comprises of a commutator handle and two indicator needles placed in vertical
alignment. In order for the block instrument to work on a double line, the station master turns his
block instrument commutator to the 'Line clear side'. This causes the electrical circuit to make
contact in such a way that the advanced starter of the dispatching station can be turned off.
Working details Take an example of a block section AB situated between two stations A and B on
a single-line section (Fig. 31.20). A train is waiting at A to enter the section AB. The procedure is
as follows.
1. The station master of station A establishes telephonic contact with the station master of station
B with the help of the block instrument and requests the station master of station B to grant a line
clear, i.e., permission so that he can dispatch train A.
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Fig. 31.20 Block section AB between stations A and B
2. Once the station master at station B has ensured that the line is clear according to the prescribed
norms, he agrees to receive the train and grants a line clear. For this, he gives a private number
and operates the block instrument of his station in a prescribed manner. The station master at
station A notes this private number and simultaneously operates his block instrument so that a 'ball
token' is extracted from the block instrument.
3. The station master at station A then allows the point to be set, lowers the signal and hands over
the 'ball token' to the driver of the train waiting at station A.
4. The station master at station B also gets the points set and lowers the signal for the line on which
the train is to be received.
5. The train then starts from station A and enters the block section AB.
6. The train reaches station B. The driver of the train hands over the ball token to the station master
of station B. After ensuring that the entire length of the train has been received, the station master
at B pockets the ball token in the block instrument. He then informs the station master at station A
of the arrival of the train on a private number as proof of the same. The points at station B are then
set as they were before and the reception signals restored to their normal positions.
7. The same procedure is repeated when the train has to enter a block section BC.
The system is absolutely safe and works on the principle of providing space intervals. Most stations
on Indian Railways work on this principle. The following are the essential features of the absolute
block system.
(a) No train is allowed to leave a station unless 'permission to approach' has been received in
advance from the block station.
(b) On double lines, permission to approach is not given until the line is clear, not only up to the
first stop signal of the next station, but also for an adequate distance beyond it.
(c) On a single line, 'permission to approach' is not given until the following conditions are satisfied.
(i) The line is clear of trains running in the same direction, not only up to the first stop signal of
the next station, but also for an adequate distance beyond it.
(ii) The line is clear of trains running in the opposite direction.
(d) When two trains are running in the same direction on the same track, permission to approach
should not be given to the second train till the entire length of the first train is within the limits of
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the home signal, the 'on' status of all the signals behind the first train has been restored, and the
line is clear, not only up to the first stop signal of the station, but also up to an adequate distance
beyond it.
Automatic block system
In the space interval system, clearing a long block section is a protracted event and the subsequent
train has to wait till the preceding train clears the entire block section. This impairs the capacity of
the section with regard to the number of trains it can clear at a time. In order to accommodate more
trains in the same section, the block section is divided into smaller automatic block sections. This
is particularly done for sections that are long and have turned into bottlenecks. The essentials of
an automatic block system on a double line are as follows.
(a) The line should be provided with continuous track circuiting.
(b) The line between two adjacent block stations may, when required, be divided into a series of
automatic block signaling sections, entry into each of which will be governed by a stop signal.
(c) The track circuits should control the stop signal governing the entry into an automatic block
signaling section in the following manner.
(i) The signal should not assume the 'off' position unless the line is clear in advance, not
only up to the next stop signal, but also for an adequate distance beyond it.
(ii) The signal should automatically turn on as soon as the train passes it.
9.9 Telecommunication
Telecommunication is a vital infrastructure for managing any transportation network. Indian
Railway has an in-house Railway Telecommunication Network for managing Train operations and
staff management and to offer Passenger Amenities.
1. Train Control Communication:
Movement of each and every train is monitored by a controller at the nearest divisional Hqrs.
Facility is also provided to the driver or guard to communicate with divisional Hqrs through
portable telephone which can be easily connected to the overhead line wires which are running
parallel to the track or connected to the Emergency Telephone sockets provided at every KM in
the section where controls are working through under ground cables. An emergency portable
telephone is kept in the Guard’s compartment of each and every train.
2. Block Circuits
Running of trains in each section (between any two stations) is controlled by block circuits through
which running of only one train in a section at one time is Electrically ensured in addition to oral
confirmation. Overhead lines of Railway or BSNL and underground cables are used for this
purpose.
3. Optical Fibre Cable network
Optical Fibre Cable is laid along the track to provide a reliable and noise free communication.
OFC network is widely used for Railway Control Communication taking advantage of its all long
haul high bandwidth circuit interconnecting Railway Telephone Exchange. Passenger Reservation
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System, Unreserved Ticketing System, Network Freight Operating Management system have been
transferred through railway OFC.
In Southern Railway distribution of various media for Telecommunication is as follows
1. OFC and RE quad cable in Electrified sections
2. OFC and 4/6 quad cable
3. Only OFC.
4. Railway owned overhead line.
5. Rented overhead line/ channels/ bandwidth from BSNL
4 Railway Telephone Network
There is an in-house Railway Telephone Network connecting all-important offices, officials, Way
stations, Divisional Headquarters & Zonal Head Quarters. Railway telephones exchanges are inter-
connected through Railway OFC network, Railway Microwave network and are supported by
rented BSNL channels as stand by.
5 Railway Microwave Communication Network
In Southern Railway telecom network is supported by Railway Owned MW network using state
of the art technology (Digital Microwave System). MW network is spread over Chennai-
Jolarpettai, Erode – Palghat, Chennai - Tiruchchirappalli, Madurai - Palghat covering all divisional
headquarters, mostly along the tracks.
6 Wireless communication System
Driver, Guard, Supervisors & officers of permanent way, Mechanical, Electrical and Signal &
Telecom departments are provided with 5 watts hand held walkie-talkies, which can be used to
establish communication between moving train & adjacent stations. Every railway station is
provided with 25 watts VHF set for this purpose.
7 Data networks
There is an exclusive PRS network connecting Chennai and all the PRS centers of Southern
Railway and other Metros. The centers are connected either through Railway OFC network or
hired channels from BSNL. Similarly, there is a Freight Operating Management System network
for monitoring the movement of freight transport. Coach Operation Information System is a
network for coach management and this is under implementation.
8 Passenger Amenities
Safety, security and comfortable journey of the passengers are the aims of Railways in train
operation. To meet this objective, the following facilities have been provided in almost all-
important stations.
• Continuous announcement through public address system
• Electronic display board
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• IVRS system for giving on line information about availability of Accommodation, arrival &
departure of trains.
• Call centers and integrated IVRS for giving all types of passenger Information.
9 Voice Recorder
Train operation information between controllers at Divisional headquarters and way stations are
normally passed through control circuits. All such conversations between section controller and
station Master are recorded at control office, which can be used for train management at any time
of investigation in case of any accident/mishap.
10 Rail net
Railway has its own data network for management purpose called “RAILNET”. This is widely
used for file transfer, e-mail and public information. Public can visit site www.gov.railnet.in. This
network spreads through entire Railway system connecting divisional headquarters, Zonal
headquarters, workshops and hospitals.
11 Disaster Management
Telecom plays a vital role in Disaster Management. To meet the requirement of Disaster
Management a universal number is provided at all control offices which can be accessed from any
part of India duly pre-fixing the city code. There are Accident Relief Trains and Medical Relief
Vans placed at strategic locations. All such ARTs and MRVs are equipped with mobile
INMARSAT telephones, walkie-talkie sets and public address system. Video conferencing
equipments and wireless satellite-based modems are also being added.
12 Video conferencing Video conferencing facilities are available in divisional headquarters, zonal
headquarters and Railway board, for administrative purpose. 2.14 Tele Medicine A wide band
connection has been established between Railway Hospital/ Perambur, Railway Hospital/Golden
Rock and with major Railway hospitals in other Zonal Railways. This enables exchange of
expertise opinion between hospitals.
9.10 Compensators
Compensators are high performance, flexible connecting elements for movement and tolerance
compensation in air conditioning and ventilation systems. Specially designed compensators are
utilized in locomotives and passenger train coaches. Extreme installation situations can be
implemented with asymmetric geometries.
9.11 Fouling marks
A fouling mark (Fig. 27.12) is provided between two converging tracks at the point beyond which
the centre-to-centre distance of the track is less than the stipulated minimum distance. This
minimum distance is 4.265 m for BG and 3.66 m for MG lines. A vehicle standing on a loop line
is not stabled beyond the fouling mark, otherwise it may have a side collision with the vehicle
standing on the main line. The salient features of a fouling mark are as follows.
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(a) A fouling mark consists of a stone or concrete block or an old wooden or steel sleeper painted
white.
(b) The fouling mark should be visible from a distance. Therefore, it is painted white and has the
letters FM marked on it in bold using black paint.
(c) The top of the fouling mark should be in line with the top of the ballast section.
(d) The fouling mark should be fixed firmly on the ground at right angles to the track.
9.12 Track capacity
The maximum number of trains which can be moved in each direction over a specified section of
track in a 24 hour period.
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