Often, automatic couplers are operated by a compressed air supply. This is used to provide power to engage and disengage the mechanical coupling and to open and close the connecting valves and contacts. Apart from automatic couplers and brakes, already mentioned above, there are a number of items on a train which require a compressed air supply for operation. Each item will have its own isolating cock to allow for maintenance and most of the larger systems have their own storage reservoir. Some are equipped with gauges as well, although most engineers prefer just a test plug.
Gauges stick out and get knocked off too easily.
Nothing drains a reservoir more quickly than a broken gauge. Although electrical operation of traction control equipment is the most common, many traction control systems use compressed air to operate circuit breakers, contractors or camshafts. There is normally a traction control reservoir and its associated isolating cock provided for each vehicle set of equipment. Doors Many rapid transit and suburban trains use air operated door systems, controlled from the cab at one end of the train but using air stored in reservoirs on each car. The reservoirs are replenished automatically by way of their connection to the main reservoir pipe.