Alignment of railway line refers
to the direction and position given to the center line of the railway track on the
ground in the horizontal and vertical planes.Horizontal alignment means the
direction of the railway track in the plan including the straight path and the curves
it follows. Vertical alignment means the direction it follows in a vertical plane
including the level track, gradients, and vertical curves.The various factors affecting
alignment, and types of track alignments and their suitability in different terrains are discussed in
this article.
Importance
A new railway line should be
aligned carefully after proper considerations, as improper alignment may ultimately
prove to be more costly and may not be able to fulfill the desired objectives.
Railway line constructions are capital-intensive projects, once constructed, it is
very difficult to change the alignment of a railway line because of the costly
structures involved, difficulty in getting additional land for the new alignment, and such other
considerations.
Requirements of an Ideal Alignment
The ideal alignment of a railway
line should meet the following requirements.
Purpose of the
New Railway Line
The alignment of a new railway
line should serve the basic purpose for which the railway line is being
constructed. As brought out earlier, the purpose may include strategic considerations,
political considerations, developing of backward areas,connecting new trade centers, and
shortening existing rail lines.
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| proposed railway alignment |
Integrated
Development
The new railway line should fit
in with the general planning and form a part of the integrated development of the country.
Economic
Considerations
The construction of the railway
line should be as economical as possible. The following aspects require special
attention.
Shortest route It is desirable
to have the shortest and most direct route between the connecting points. The
shorter the length of the railway line, the lower the cost of its construction, maintenance,
and operation. There can, however, be other practical considerations that can
lead to deviation from the shortest route.
Construction and
maintenance cost The
alignment of the line should be so chosen that the construction cost is
minimum. This can be achieved by a balanced cut and fill of earthwork, minimizing
rock cutting and drainage crossings by locating the alignment on watershed lines, and
such other technical considerations. Maintenance costs can be reduced by avoiding
steep gradients and sharp curves, which cause heavy wear and tear of rails and
rolling stock.
Minimum
operational expenses The
alignment should be such that the operational or transportation expenses are
minimum. This can be done by maximizing the haulage of goods with the given
power of the locomotive and traction mix. This can he achieved by providing easy
gradients, avoiding sharp curves, and adopting a direct route.
Maximum Safety
and Comfort
The alignment should be such that
it provides maximum safety and comfort to the travelling public. This can be
achieved by designing curves with proper transition lengths, providing vertical
curves for gradients, and incorporating other such technical features.
Aesthetic
Considerations
While deciding the alignment,
aesthetic aspects should also be given due weightage.A journey by rail should be
visually pleasing. This can be done by avoiding views of borrow pits and passing the
alignment through natural and beautiful surroundings with scenic beauty.
Selection of a Good Alignment
Normally, a direct straight route
connecting two points is the shortest and most economical route for a railway
line, but there are practical problems and other compulsions which necessitate
deviation from this route. The various factors involved is the selection of a good alignment for a railway
line are given below.
Choice of Gauge
The gauge can be a BG (1676 mm),
an MG (1000 mm), or even an NG (762 mm).As per the latest policy of the
Government of India, new railway lines are constructed on BG only.
Obligatory or
Controlling Points
These are the points through
which the railway line must pass due to political,strategic, and commercial reasons
as well as due to technical considerations. The following are obligatory or
controlling points.
Important cities
and towns These
are mostly intermediate important towns, cities,or places which of commercial,
strategic, or political importance.
Major bridge
sites and river crossings The construction of major bridges for large rivers is very expensive
and suitable bridge sites become obligatory points for a good alignment.
Existing passes
and saddles in hilly terrain Existing passes and saddles should be identified for crossing a
hilly terrain in order to avoid deep cuttings and high banks.
Sites for
tunnels The
option of a tunnel in place of a deep cut in a hilly terrain is better from the economical
viewpoint. The exact site of such a tunnel becomes an obligatory point.
Topography of
the Country
The alignment of a new railway
line depends upon the topography of the country it traverses. The following few
situations may arise.
Plane alignment When the
topography is plane and flat, the alignment presents no problems and can pass through
obligatory points and yet have very easy gradients.
Valley alignment
The
alignment of a railway line in valley is simple and does not pose any problem. If two control
points lie in the same valley, a straight line is provided between these points
with a uniform gradient.
Cross-country
alignment The
alignment of a railway line in such terrain crosses the watersheds of two or more
streams of varied sizes. As the levels vary in crosscountry,the gradients are steep and
varying and there are sags and summits. The controlling or obligatory points
for cross-country alignment may be the lowest saddles or tunnels. It may be
desirable to align the line for some length along the watersheds so that some of the
drainage crossings may be avoided.
Mountain
alignment The
levels in mountains vary considerably, and if normal alignment is adopted, the grades
would become too steep, much more than the ruling gradient (allowable
gradient). In order to remain within the ruling gradient,the length of the railway line is increased
artificially by the ‘development process’.
Geometrical
Standards
Geometrical standards should be
so adopted as to economize as much as possible as well as provide safety and
comfort to passengers. This can be done by adopting gradients and curves within
permissible limits. Transition as well as vertical curves should be used to provide better comfort and safety.
Geological
Formation
The alignment should be so
selected that it normally runs on good and stable soil formation as far as possible.
Weak soil and marshy land present a number of problems including those of
maintenance. Though rocky soil, provides a stable formation, it is a costly proposal.
Effect of Flood
and Climate
The alignment should normally
pass through areas which are not likely to be flooded.The climatic conditions should
also be taken into consideration for alignment. In hot climate and sandy areas, the
alignment should pass by those sides of sand dunes that face away from the
direction of the wind. Similarly, in cold regions, the alignment should pass by those
sides of hills that face away from the direction of the wind. A sunny
side is more desirable.
Position of
Roads and Road Crossings
A railway line should cross a
road at right angles so as to have a perpendicular level crossing and avoid
accidents.
Proximity of
Labour and Material
The availability and proximity of
local labour and good and cheap building material should also be considered when
deciding the alignment.
Location of
Railway Stations and Yards
Railway stations and yards should
be located on level stretches of land, preferably on the outskirts of a town or
village so as to have enough area for the free flow of traffic.
Religious and
Historical Monuments
The alignment should avoid
religious and historical monuments, as it is normally not possible to dismantle these
buildings.
Cost
Considerations
The alignment should be such that
the cost of construction of the railway line is as low as possible. Not only the
initial cost of construction but also the maintenance cost should be as low as
possible. For this purpose, the alignment should be as straight as possible, with least
earthwork, and should pass through terrain with good soil.
Traffic
Considerations
The alignment should be so
selected that it attracts maximum traffic. In this context,traffic centers should be well
planned; so that the railway line is well patronized and the gross revenue arising out
of traffic receipts is as high as possible.
Economic
Considerations
Keeping in mind the various
considerations, it should be ensured that the alignment is overall economical. For this
purpose, various alternate alignments are considered and the most economical one,
which is cost effective and gives the maximum returns is chosen.
The maximum annual return (ƛ) is
calculated by the formula
ƛ = ( R-E ) / I
where R is the gross
revenue earned by the railway line and E denotes the annual running expenses.It may be noted here that R depends
upon the route that proves to be advantageous when taking traffic into
consideration and, therefore, should be given due weightage.The other way to maximize the
annual return is to have sound and economical construction work so as to reduce
the annual running expenses. A suitable balance has to be
achieved between construction cost and
operating expenses.
Political
Considerations
The alignment should take into
account political considerations. It should not enter foreign soil and should preferably be away from
common border areas.
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