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.

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:

Superelevation on Curves (Cant)

Cant is defined as the difference in height between the inner and outer rails on the curve. It is provided by gradually raising the outer rail above the inner rail level. The inner rail is considered as the reference rail and normally is maintained at its original level. The inner rail is known as the gradient rail.

Function of superelevation:

– Neutralizes the effect of lateral force.

– It provides better load distribution on the two rails.

– It reduces wear and tear of rails and rolling stock.

– It provides smooth running of trains and comforts to the passengers.

Speeds

Equilibrium speed: It is the speed at which the effect of centrifugal force is exactly balanced by the superelevation provided. It can also be said that when the speed of a vehicle running on a curved track is such that the resultant weight of the vehicle and the effect of radical acceleration is perpendicular to the plane of rails and the vehicle is not subjected to an unbalanced radical acceleration, is in equilibrium then its particular speed is called equilibrium speed.

Maximum permissible speed: This is the highest speed which may be allowed or permitted on a curved track taking into consideration of the radius of curvature, actual cant, cant deficiency, cant excess and the length of the transition curve. When, the maximum permissible speed on the curve is less than the maximum sanctioned speed of the section of a line, permanent speed restriction become necessary on such curves.

Cant deficiency

Cant deficiency is the difference between the equilibrium cant (theoritical) necessary for the maximum permissible speed on a curve and the actual cant provided there. As per Indian Railways, Cant deficiency is recommended as follow: 

•   BG Track:          75 mm
•   MG track:          50 mm
•   NG track:          40 mm

Cant Excess

When a train travels on a curved rack at a speed lower than the equilibrium speed, then the cant excess occurs. It is the difference between the actual cant provided and the theoretical cant required for such lower speeds. Maximum value for cant excess is

• BG track:       75 mm
• MG Track:      65 mm

Centrifugal Force

When a body moves on a circular curve, it has a tendency to move in a straight direction tangential to the curve. This tendency of the body is due to the fact that the body is subjected to a constant radial acceleration.