Chapter 3:

Alignment of Railway Lines

Introduction

 The direction and position given to the centre line of the railway track on the ground is
called the track alignment.
 The horizontal alignment includes the  The vertical alignment of a railway track
includes changes in
o straight path,
o gradients and
o its width,
o vertical curves.
o deviations in width and
o horizontal curves.
 Basic Requirements of Good Track Alignment

In case of railways, the profit is not the only objective but comfort to the passengers must be given
equal importance.

Though the income from transportation of goods is much more than that from passenger traffic but
the safety, comfort and convenience of passengers are always kept in view.

An ideal alignment should fulfil the following requirements –

1. Purpose of Track
 The alignment of the track should be done keeping in
view the basic purpose or purposes, it is going to
serve.

 In general, the railways serve the following purposes –

(i) Transportation Services

 Railways carry the passenger traffic, and goods
traffic, local as well as through.
(ii) Political and Strategic
 Sometimes it becomes essential to construct a railway
line to connect
o two points (either with the neighbouring countries or
within the country) for defence purposes
o so that in case of emergency, armies within the
country can be transferred from one place to another.

(iii) Linking of Centres

 When a railway line is constructed basically
o to connect important places for providing transport
services, it may be linking of two trade centres.
o In such cases, a new railway line is proposed to
connect two trade centres.
(iv) To Open up New Tracks

 It may be necessary to align new

tracks for the land, whose resources
are not yet tapped.

(v) Shortening Existing Track

 When an existing track is zig-zag or

uneconomical, a new track may be
aligned to shorten the existing track
which would prove to be economical.
2. Feasibility 3. Economy
The track alignment will be economical with
For aligning a railway line,
consideration of the following factors –
 it is necessary to carryout feasibility
1. Shortest route
study so that the proposed track alignment
meets
 shortest and the most direct route between two
 all technical requirements also
joining points is cheapest, though, there may be
 fits in well with the general planning of  several practical considerations which would
the country.
cause a deviation from shortest route.
2. The initial construction cost should be minimum.

 This can be achieved by avoiding

o loose earth slopes,
o rock-cuttings,
o drainage crossings by aligning the track on
watersheds, etc., if feasible.

Consequence of no/poor
treatment:
Landslide hits rail services
in Odisha's Koraput:
Published in : The New
Indian Express
3. Maintenance cost should be minimum.
 This is more important than the construction cost, being
recurring in nature.
 Maintenance cost can be reduced by avoiding
o deep cutting,
o very high banks,
o long viaducts,
o tunnels and
o heavy gradients which cause heavy wear on rails and
rolling stock.
4. The operating expenses or transportation cost should be minimum.

 The transportation cost will be minimum, when the haulage

of goods is maximum for the given power of locomotive and
type of traction.

 This can be achieved by providing

o easy gradients,
o avoiding unnecessary rises and
o adopting shortest-direct-route.
4. Safety
 The track should be so aligned that it gives
o safety to traffic operations
o passengers and goods traffic can be transported
without any chance of accident or derailment.
 To achieve this, the track should be properly designed
from the viewpoint of
o curves,
o gradients and
o at the same time stability of natural slopes,
embankment and cut-slopes.
o Foundations of embankments should be properly
maintained.
To achieve this, the track should be properly designed from
the viewpoint of
o curves,
o gradients and
o at the same time stability of natural slopes,
embankment and cut-slopes.
o Foundations of embankments should be properly
maintained.
5. Aesthetic Aspects

 This aesthetic aspect of a railway line should be kept in view for comfortable and
pleasant railway journey.

 This is achieved by avoiding the

o view of borrow pits,

o use of transition curves, etc., or

o keeping the track through beautiful natural surroundings.

Factors in selection of good track alignment
 Though the direct shortest route is the most economical but is
rarely possible due to various practical difficulties such as
 intermediate obstructions,
 steep gradients within the shortest route,
 construction and maintenance problems etc.
 Moreover, it may be necessary to deviate from the shortest
route to connect obligatory points. (i.e. traffic generating points
or places of importance).
 An alignment, which is economical in the initial cost, may not
prove most economical in maintenance or vehicle operation
cost or both.

 It is also possible that shortest route may be costlier than

other routes, when different alternatives are tried from
construction point of view.

 Thus, it may be concluded that all the requirements cannot

be fully justified simultaneously.
For satisfying most of the requirements, the following factors in selection of good alignment,
require due consideration –

 Obligatory or controlling points

 Position, Amount and Type of Traffic

 Gauge-Selection

 Geometric Standards

 Topography of the country

 Economic Considerations

 Other Considerations
1. Obligatory or Controlling Points
These are the points which govern the alignment of a railway track. These points can be mainly
classified into two categories –
(i) Points through which a track must pass (ii) Points through which a track should not pass
(a) Important Towns and Cities (a) Acquisition of costly level which will result in

(social, commercial, political and defence importance) heavy compensation should be avoided.

(b) Major-Bridges or River Crossings (b) Religious places like temple, church, mosque
or tomb should be avoided as they are
(costly affair so the bridge sites is very important)
protected by law.
(c) Hill-passes
(c) Areas liable to flooding (i.e., water logged
(avoid unnecessary deep cuttings or high banks, or areas), marshy areas, cutting in snowfall
tunnels, the existing passes). areas or such unsuitable areas of alignment

(d) Site for Tunnels should be avoided.

(passing of a track through tunnel should be avoided)

2. Traffic – Its Position, Nature and Amount
• The position of traffic sources furnishes control
points for general location of the alignment.

• The nature of traffic (passenger or goods

traffic) and potential volume of traffic govern
the type of construction to justify the revenues.

• So it is essential to estimate the growth of

traffic, its position, nature and amount before
aligning the track.
Traffic varies as the square of the
population. Therefore, the route, which has
the greatest population, is the best.

Freight earnings are much more than

passenger earnings.

Whenever a new line is aligned, all the

traffic from an influence area upto a width
of 15 km on either side would go to the So while conducting traffic surveys all the prospects
railway line in the beginning and after few of traffic-growth and impact within influence area
years this width increases to 25 km. should be studied before aligning any track.
Gauge Selection
In brief, it can be said that though the increase in width of gauge increases initial cost but it also
increases load carrying capacity and the speed of the trains.

Geometric Standards (economical combination of construction and operation costs).

 Performance of locomotives depends upon gradients, speed, loading and method of traction used.
 Ruling gradient and minimum permissible radius of the curve must be considered
 Lesser the radius of curve more would be the operating expenses due to curve resistance.
 The use of curves should be particularly avoided on approaches and at stations as well as on
bridges.
 All the large bridges should be crossed by the alignment at right angles and by use of at least 30 m
straight portion on either side of the bridge.
 The effect of rise and fall of ground should be considered, as they affect the cost of alignment and
operative expenses.
 The effect of distance on operating expenses should be considered in the alignment selection.
 The railway passenger rates and freight charges are fixed according to distance hauled.
 The use of steep gradients increases the operating costs considerably.
Topography of the Country
If topography of country is such that the use of heavy gradients is
unavoidable then in such cases the alignment of the track is made
by the special ways to reduce the rate and cost of high-gradients.
For alignment, the topography can be classified under the following
heads –

 Valley alignment
 Cross-country-alignment
 Mountain alignment

(i) Valley Alignment

• This alignment as such creates no problems.

• If two control points lie in the same valley, the

engineers should choose a straight line between
terminal points with a uniform rate of gradient.

• The ends having steeper gradients can be further

negotiated by some form of development.
(ii) Cross Country Alignment
 This alignment crosses the water-sheds of two or more streams of different sizes.

 Unlike the valley alignment, the uniform grade is not possible because the rail levels are so
high that there are no appreciable sags or summits between bridges.

 So the routes in cross-country have sags and summits in succession.

 The lowest saddles or short tunnel lines may work as the controlling or obligatory points.
(iii) Mountain Alignment
• This type of alignment is best obtained by development
accompanied by tunneling.

• For this development, the length of alignment is first

increased between the two extreme ends of elevation so
as to limit the gradient maximum up to ruling gradient.

• This development is accomplished by the following

special ways –

(a) By Zig-Zag Development

o In this, at a favourable point the line makes a half circle

loop and returns upon itself in order to gain height at
the expense of losing distance.

o This line is in succession of such loops up the side-

valleys or round the hill side alternatively with the
principle of maintaining convenient side-slope which
is at right-angles to the direction of alignment.
(b) By Switch-Back Development

• In case of very steep-side slopes a considerable gain is

accomplished by switch-backs.

• In Fig, A and B are the switches and C and D are buffer

stops.

• The vertical curve is provided to bring the train to a stop

slowly and gradually without the use of brakes.

• So the train stops at switch point and hence the switches

preferably should be located in a station yard.
(c) By Spirals or Complete Loops Development – A spiral is a
doubling back of the line upon itself, round the sides of a valley
or basin of a hill, so that it crosses and returns under-itself at a
lower elevation.

Spirals are of two types –

 A bridge-spiral swings around the slopes of a valley where a
bridge or viaduct can span the valley at considerable height.
So bridge-spiral can be used for a narrow valley.
A tunnel spiral curves round the slopes of a hill. This
gives more favourable gradients than in descending
with the slope of the valley on one side only as in
case of zig-zag development.
6. Economic Considerations
Keeping in view the above factors, the alignment should be economical also.
For this purpose, different alternatives of alignment keeping in view the basic purposes should be
analysed from cost point of view, The best alignment would be one, which gives the maximum annual
return (r) given by the formula

This shows ‘r’ should be maximum which depends upon the proper selection of the route so as to
include the advantages of traffic generating centres.

The other way of increasing the annual returns is sound track construction which reduces the
annual running expenses. So a suitable balance between the construction cost and operating
expenses should be stack.

The biggest item of expenditure is the maintenance of the track, so a reasonable economy in
selection of materials and construction should be made.
7 Other Considerations
(i) While aligning the track Geological formation should be studied for the following reasons :
 The alignment should be made on good soil.
 Alignment should be so laid that it is free of slips or slides.
 The alignment should not be on rocky soil (costly excavation and possibility of embankment to slip.
(ii) The effect of flood and climate on the track alignment is important due to following reasons :
 Waterlogged areas should be avoided.
 Cutting in snowfall areas should be avoided as it results in trouble of clearing the track all the time.
 In cold region, the track should be aligned on that side of hill which is not affected by winds. A sun
facing side is desirable.
 The track should cross the road wherever level crossing is there at right angles to avoid accidents.
 The station sites on the track should be selected outside the town or city. It should be on a level
stretch of ground.
 The track should pass, if possible, in the proximity of rural areas or villages. Because the stones are
easily available from quarries for construction and maintenance, and labour is cheaply available from
the villages.