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. The various gauges have been discussed in detail in Chapter 2.

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'. The following are the standard methods for the development technique:

Zigzag line method In this method, the railway line traverses in a zigzag alignment (Fig. 3.1) and follows a convenient side slope which is at nearly right angles to the general direction of the alignment. The line then turns about 180° in a horseshoe pattern to gain height.

Switch-back method In the case of steep side slopes, a considerable gain in elevation is accomplished the switch-back method (Fig. 3.2). This method involves a reversal of direction achieved by a switch, for which the train has to necessarily

stop. The switch point is normally located in a station yard. In Fig. 3.2, A and B are two switches and A1 and B1 are two buffer stops. A train coming from D will stop at B1 and move in back gear to line BA. It will stop at A1 again and then follow the line AC.

Switch-back alignment
Fig. 3.2 Switch-back alignment

Spiral or complete loop method This method is used in a narrow valley where a small bridge or viaduct has been constructed at a considerable height to span the valley (Fig. 3.3). In this case, normally a complete loop of the railway line is constructed, so that the line crosses the same point a second time at a height through a flyover or a tunnel.

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 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.

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 (g) is calculated by the formula

R - E g = -

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.

Basic Requirements of an Ideal Alignment | RAILWAY ENGINEERING - Contents | Mountain Railways