How does a brake cylinder function in an air brake system?

The brake cylinder plays a crucial role in an air brake system by converting air pressure into mechanical force that effectively applies the brakes. When air is introduced into the brake cylinder, the pressure generated from this air causes a piston within the cylinder to move. This movement is what translates the air pressure into the necessary force needed to engage the brake shoes against the wheels, thereby slowing down or stopping the train.

This process is fundamental to the operation of the air brake system, as the precise application of force allows for controlled and efficient braking. Understanding this function is key for anyone working with or studying air brake systems, as it highlights how air pressure directly impacts braking performance. The other choices do not accurately describe the primary function of the brake cylinder or its operational context within the air brake system.