This Article Covers Gradient In Railway Tracks Their Types And Geometric Design One By One.
Gradient In Railway
Geometric design of a railway track discusses all those parameters which affect the geometry of the track. These parameters are as follows:
- Gradients in the track, including grade compensation, rising gradient, and falling gradient
- 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.
- Alignment of the track, including straight as well as curved alignment
It is very important for tracks to have proper geometric design in order to ensure the safe and smooth running of trains at maximum permissible speeds, carrying the heaviest axle loads.
The speed and axle load of the train are very important and sometimes also included as parameters to considered while arriving at the geometric design of the track.
Necessity For Geometric Design
The need for proper geometric design of a track arises because of the following considerations
- To ensure the smooth and safe running of trains
- To achieve maximum speeds
- And To carry heavy axle loads
- To avoid accidents and derailments due to a defective permanent way
- To ensure that the track requires least maintenance
- For good aesthetics
Details Of Geometric Design Of Track
The geometric design of the track deals with alignment of railway track and Curves Details regarding curves and their various aspects.
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 movement of traffic and in a down or falling gradient the track loses elevation the direction of movement of traffic.
A gradient normally represented by the distance travelled for a rise or fall of one unit.
Sometimes the gradient 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 per cent.
Gradients are provided to meet the following objectives
- To reach various stations at different elevations
- To follow the natural contours of the ground to the extent possible
- And To reduce the cost of earthwork
Types Of Gradients
The following types of gradients used on the railways
- Ruling gradient
- Pusher or helper gradient
- Momentum gradient
- Gradients in station yards
The ruling gradient is the steepest gradient that exists in a section. It determines the maximum load that can 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 taken into consideration.
The power of the locomotive to 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.
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.
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 not adequate to pull the entire load, and an extra locomotive required.
When the gradient of the ensuing section so steep as to necessitate the use of an extra engine for pushing the train, it known as a pusher or helper gradient.
Examples of pusher gradients are the Budni-Barkhera section of Central Railway and the Darjeeling Himalayan Railway section.
The momentum gradient also steeper than the ruling gradient and can overcome by a train because of the momentum it gathers while running on the section. In valleys, a falling gradient 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.
And 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.
Gradients in Station Yards
The gradients in station yards are quite flat due to the following reasons
- It prevents standing vehicles from rolling and moving away from the yard due to the combined effect of gravity and strong winds.
- And It reduces the additional resistive forces required to start a locomotive to the extent possible.
It may mentioned here that generally, yards 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 compensated to the following extent on curves:
- On BG tracks, 0.04 per cent per degree of the curve or 70/R, whichever is minimum
- On MG tracks, 0.03 per cent per degree of curve or 52.5/R, whichever is minimum
- And On NG tracks, 0.02 per cent 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.
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