The reservoirs are replenished automatically by way of their connection to the main reservoir pipe. Door systems usually use lower than normal MR air pressure. Air Suspension Placing the car body on air pressure springs instead of the traditional steel springs has become common over the last 20 years for passenger vehicles. The air spring gives a better ride and the pressure can be adjusted automatically to compensate for additions or reductions in passenger loads. The changes in air pressure are used to give the brake and acceleration equipment the data needed to allow a constant rate according to the load on the vehicle.
Most trains use compressed air for brake operation.
Most locomotives and older EMU's use a pneumatic brake control system which requires a brake valve to be operated by the driver. The valve controls the flow of air into and out of the brake pipe which, in turn, controls the brakes on each vehicle in the train consist. The driver's brake valve is connected to the MR pipe in the cab so that there is always a constant supply of air available to replenish the brake control system when required. An isolating cock is provided in the cab so that the brake control can be closed off when the cab is not in use. Pantograph Compressor One additional compressor is often provided on units which have air operated pantographs, i.e. those which are raised or lowered using compressed air as the power medium.