Explanation:
In railways, gradients are used to describe the slope or incline of a railway track. Gradients are important in determining the efficiency and safety of train operations. There are different types of gradients used in railways, but one type that is not used is the ascending gradient.

Ascending Gradient:
An ascending gradient refers to a gradient that goes uphill or has an upward slope. It means that the track is inclined in the direction of travel. Ascending gradients are used in railways when trains need to climb uphill or gain elevation. Trains require more power to move uphill against gravity, and the ascending gradient helps in providing the necessary traction.

Momentum Gradient:
A momentum gradient refers to a gradient that allows trains to gain momentum or speed without using excessive power. It is a relatively flat or slightly downhill gradient that allows trains to maintain their speed or accelerate with minimal power input. Momentum gradients are used to conserve energy and improve the efficiency of train operations.

Push Gradient:
A push gradient refers to a gradient that assists trains in moving uphill by utilizing external pushing forces. It is a type of gradient where another locomotive or engine pushes the train from behind, providing additional power to overcome the uphill slope. Push gradients are used in situations where the primary locomotive is not powerful enough to climb the gradient on its own.

Control Gradient:
A control gradient refers to a type of gradient that is controlled or limited to ensure the safe and efficient operation of trains. It is used to regulate the speed and prevent excessive acceleration or deceleration. Control gradients are typically used in areas with sharp curves, steep gradients, or restricted visibility.

Conclusion:
Among the given options, the ascending gradient is the type of gradient that is not used in railways. Ascending gradients are not used because they require more power and can pose challenges for trains to climb uphill. Other types of gradients, such as momentum gradients, push gradients, and control gradients, are used to improve train operations, conserve energy, and ensure safe travel.