There are few hills or mountains that have been connected by railway lines. Some of the important features of these mountain railways are the following.
Gradients The gradients of mountain tracks are very steep. Normally tracks with gradients of 3% or more are considered mountain tracks.
Gauge Normally narrow gauges with gauge widths of 762 mm or 610 mm are adopted for mountain railways.
Curvature The curvature of mountain tracks is very sharp. Curvatures of up to 40 o are normally adopted.
Alignment Mountain alignment is quite zigzag and not straight in order to gain heights easily. The type of alignments normally followed are zigzag, switch-back, and spiral.
Purpose Mountain railways have mostly been constructed for tourist traffic. In some cases, they may be constructed for exploiting hinterland or new areas.
Cost considerations The cost of construction of mountain railways is quite high because of the need for a large number of bridges and heavy earthwork. The cost, however, gets considerably reduced by adopting a smaller gauge, narrow gauge (0.762 m or 0.61 m), and then it is possible to have very steep gradients and sharp curvature.
Some of the important mountain railways existing in India today are: (i) Kalka- Shimla Railway, (ii) Nilgiri Mountain Railway, (iii) Matheran Light Railway, (iv) Darjeeling Himalayan Railway, (v) The Kangra Valley Railway, and (vi) Haldwani-Kathgodam Railway.
In the case of very steep gradients, much steeper than 3%, it becomes difficult for a locomotive to pull the train load and hence is possible for the train slide down or slip back along the down grade.
The rack railway system consists of three rails, i.e., one extra toothed rail in the middle in addition to two normal rails. The locomotive also has a toothed pinion wheel whose teeth fit into the grooves of the central toothed rail. This locking arrangement helps to haul the train load and does not let the locomotive slip back. There are various such systems existing in the Indian Railways, such as the Fell system, Abt system, Riggenbach system, and Marsh system, each with its own characteristics.
With the help of this rack and pinion system, it is possible to move trains even on very steep gradients such as 1 in 5.