Pusher gradient, also known as helper gradient, is a term used in railway engineering to refer to an additional gradient provided to assist trains in climbing steep gradients. It is an important concept in the design and operation of railway track systems.

Explanation:
The pusher gradient is a gradient that is steeper than the ruling gradient, which is the maximum gradient that a train can ascend or descend without the assistance of additional power. The pusher gradient allows trains to climb or descend slopes that are steeper than the ruling gradient by providing additional power to overcome the extra resistance.

Importance:
The pusher gradient is necessary in situations where the ruling gradient is not sufficient to overcome the resistance and maintain the desired speed. It helps prevent trains from stalling or losing speed on steep gradients, ensuring efficient and reliable operation of the railway system.

Function:
The pusher gradient can be achieved by various means, such as using additional locomotives or applying extra power to the existing locomotives. This additional power helps to compensate for the increased resistance on steep gradients, allowing trains to maintain their desired speed and complete the journey safely and efficiently.

Advantages:
- Enables trains to climb or descend steep gradients that would otherwise be impossible or difficult to traverse.
- Ensures efficient and reliable operation of the railway system by preventing stalling or loss of speed on steep gradients.
- Increases the capacity and flexibility of the railway network by allowing trains to operate on routes with challenging gradients.

Limitations:
- Requires additional power resources, such as extra locomotives or increased power supply, which can add to the operational costs.
- May require special infrastructure modifications, such as additional tracks or sidings, to accommodate the extra locomotives or power supply.

In conclusion, the pusher gradient, also known as the helper gradient, is an additional gradient provided to assist trains in climbing or descending steep slopes. It is an important concept in railway engineering as it ensures efficient and reliable operation of the railway system on challenging gradients.