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Details of Geometric Design of Track
The geometric design of the track deals with the various aspects described in the following.
Alignment of railway track The subject of railway track alignment has been covered in Previous Pages.
Curves Details regarding curves and their various aspects have been discussed in Previous Pages.
Gradients Details regarding gradients are discussed in the following section.
Gradients of Track
Gradients are provided to negotiate the rise or fall in the level of the railway track. A rising gradient is one in which the track rises in the direction of the movement of traffic and a down or falling gradient is one in which the track loses elevation in the direction of the movement of traffic.
A gradient is normally represented by the distance travelled for a rise or fall of one unit. Sometimes the gradient is indicated as per cent rise or fall. For example, if there is a rise of 1 m in 400 m, the gradient is 1 in 400 or 0.25%, as shown in Fig. 12.1.
Gradients are provided to meet the following objectives.
(a) To reach various stations at different elevations
(b) To follow the natural contours of the ground to the extent possible
(c) To reduce the cost of earthwork.
The following types of gradients are used on the railways.
(a) Ruling gradient
(b) Pusher or helper gradient
(c) Momentum gradient
(d) Gradients in station yards
1 Ruling Gradient
The ruling gradient is the steepest gradient that exists in a section. It determines the maximum load that can be hauled by a locomotive on that section. While deciding the ruling gradient of a section, it is not only the severity of the gradient but also its length as well as its position with respect to the gradients on both sides that have to be taken into consideration. The power of the locomotive to be put into service on the track also plays an important role in taking this decision, as the locomotive should have adequate power to haul the entire load over the ruling gradient at the maximum permissible speed.
The extra force P required by a locomotive to pull a train of weight W on a gradient with an angle of inclination q is
P = W Sin q
= W tan q (approximately, as q is very small) = W × gradient
Indian Railways does not specify any fixed ruling gradient owing to enormous variations in the topography of the country, the traffic plying on various routes, and the speed and type of locomotive in use on various sections. Generally, the following ruling gradients are adopted on Indian Railways when there is only one locomotive pulling the train.
In plain terrain: 1 in 150 to 1 in 250 In hilly terrain: 1 in 100 to 1 in 150
Once a ruling gradient has been specified for a section, all other gradients provided in that section should be flatter than the ruling gradient after making due compensation for curvature.
2 Pusher or Helper Gradient
In hilly areas, the rate of rise of the terrain becomes very important when trying to reduce the length of the railway line and, therefore, sometimes gradients steeper than the ruling gradient are provided to reduce the overall cost. In such situations, one locomotive is not adequate to pull the entire load, and an extra locomotive is required.
When the gradient of the ensuing section is so steep as to necessitate the use of an extra engine for pushing the train, it is known as a pusher or helper gradient. Examples of pusher gradients are the Budni-Barkhera section of Central Railways and the Darjeeling Himalayan Railway section.
3 Momentum Gradient
The momentum gradient is steeper than the ruling gradient and can be overcome by a train because of the momentum it gathers while running on the section. In valleys, a falling gradient is sometimes followed by a rising gradient. In such a situation, a train coming down a falling gradient acquires good speed and momentum, which gives additional kinetic energy to the train and allows it to negotiate gradients steeper than the ruling gradient. In sections with momentum gradients there are no obstacles provided in the form of signals, etc., which may bring the train to a critical juncture.
4 Gradients in Station Yards
The gradients in station yards are quite flat due to the following reasons.
(a) To prevent standing vehicles from rolling and moving away from the yard due to the combined effect of gravity and strong winds.
(b) To reduce the additional resistive forces required to start a locomotive to the
It may be mentioned here that generally, yards are not levelled completely and certain flat gradients are provided in order to ensure good drainage. The maximum gradient prescribed in station yards on Indian Railways is 1 in 400, while the recommended gradient is 1 in 1000.
Grade Compensation on Curves
Curves provide extra resistance to the movement of trains. As a result, gradients are compensated to the following extent on curves
(a) On BG tracks, 0.04% per degree of the curve or 70/R, whichever is minimum
(b) On MG tracks, 0.03% per degree of curve or 52.5/R, whichever is minimum
(c) On NG tracks, 0.02% per degree of curve or 35/R, whichever is minimum where R is the radius of the curve in metres. The gradient of a curved portion of the section should be flatter than the ruling gradient because of the extra resistance offered by the curve.
Example Find the steepest gradient on a 2 o curve for a BG line with a ruling gradient of 1 in 200.
(i) Ruling gradient = 1 in 200 = 0.5%
(ii) Compensation for a 2 o curve = 0.04 × 2 = 0.08%
(iii) Compensated gradient = 0.5 - 0.08 = 0.42% = 1 in 238 The steepest gradient on the curved track is 1 in 238.
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