Alignment of railway line refers to the direction and position given to the centre 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.
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 fulfil 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 centres, and shortening existing rail lines.
The new railway line should fit in with the general planning and form a part of the integrated development of the country.
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.
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 cross-country 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 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.
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.
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.
The alignment should be so selected that it attracts maximum traffic. In this context, traffic centres 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.
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.
The alignment should take into account political considerations. It should not enter foreign soil and should preferably be away from common border areas.