GEOMETRY OF RAILWAY TRACK

1.1 What is track geometry?

Track geometry is three-dimensional geometry of track layouts and associated measurements used in design, construction and maintenance of railroad tracks. The subject is used in the context of standards, speed limits and other regulations in the areas of track gauge, alignment, elevation, curvature and track surface.

Although, the geometry of the tracks is three-dimensional by nature, the standards are usually expressed in two separate layouts for horizontal and vertical.

The geometric design of a railway track includes all those parameters which determine or affect the geometry of the track. These parameters are as follows.

1. Gradients in the track, including grade compensation, rising gradient, and falling gradient.
2. Curvature of the track, including horizontal and vertical curves, transition curves, sharpness of the curve in terms of radius or degree of the curve, cant or superelevation on curves, etc.
3. Alignment of the track, including straight as well as curved alignment.
4. Speed of train.
5. Widening of gauge on curves.

1.1.1 Alignment

Alignment of railway line refers to the direction and position given to the centerline 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.

1.1.2 Curves

Curves are introduced on a railway track to bypass obstacles, to provide longer and easily traversed gradients, and to pass a railway line through obligatory or desirable locations. Horizontal curves are provided when a change in the direction of the track is required and vertical curves are provided at points where two gradients meet or where a gradient meets level ground.

1.1.3 Gradients

Any departure of track from the level is known as grade or gradient.

Purpose of providing gradient:

• To provide uniform rate of rise or fall,
• To reduce cost of earth work,
• To reach different stations at different level.

1.1.3.1 Types of gradients

1. Ruling gradient: The steepest gradient allowed on the track section. It determines the max load that the locomotive can haul that section. The steep gradient needs more powerful locomotives, smaller train loads, lower speed, resulting in costly hauling.                                                                                               –In plains: 1 in 150 to 1 in 200                                                                                   –In hilly regions: 1 in 100 to 1 in 150.
2. Momentum Gradient: The gradient on a section which are steeper than the ruling gradient acquire sufficient momentum to negotiate them are known as momentum gradient.
3. Pusher gradient: As stated above a ruling gradient limits the maximum weight of a train which can be hauled over the section by a locomotive. If the ruling gradient is so severe on a section that it needs the help of extra engine to pull the same load than this gradient is known as pusher of helper gradient. In Darjeeling Railways 1 in 37 pusher gradient is used on Western Ghat BG Track.
4. Gradient at stations: at stations gradient are provided sufficient low due to following reason:                                                                                                                 –To prevent movement of standing vehicle                                                               –To prevent additional resistance due to grade.                                                       On Indian railways, maximum gradient permitted is 1 in 400 in station yards.