What supplies the force necessary to release the applied brakes on a subway car?

The force necessary to release the applied brakes on a subway car is primarily supplied by brake springs. When brakes are applied, the system uses various mechanisms to ensure that they stay engaged; however, to release them, the energy stored in the brake springs plays a crucial role. These springs are designed to automatically push the brake shoes away from the wheels when the braking system is deactivated.

This mechanism allows for a quick and efficient release of the brakes, enabling the subway to resume movement smoothly. The design ensures that once the brakes are no longer required, the springs can act rapidly to lift the braking force, allowing the subway car to accelerate or continue its journey safely.

Other options, such as air pressure and hydraulic systems, might be involved in the braking process itself, but they are not responsible for releasing the brakes in this context. Manual control does allow operators to engage or disengage the brake systems, but it does not provide the intrinsic force necessary for the release; that function relies on the mechanical energy of the brake springs.