Westinghouse Automatic Air Brake

The Westinghouse automatic two pipe air brake was universal on Southern Electric Multiple Units until 1949.

This system consisted of a main reservoir of compressed air on each motor coach, fed by an electrically-operated reciprocating compressor controlled by a pressure-sensitive governor. The main reservoirs throughout a train were linked by a main air pipe and this, in turn, was linked on each vehicle through a branch pipe and triple valve to a smaller auxiliary reservoir.

Air from the main reservoir pipe passed through the driver's brake valve into the train brake pipe.

When the driver moved the brake handle to a brake application position, the connection between the main air pipe and the train brake pipe was closed and air was allowed to escape from the train brake pipe, reducing its pressure.

The auxiliary reservoir on each vehicle was linked to a brake cylinder through a triple valve that was actuated by the train brake pipe. When the valve sensed a reduction in pressure in the train brake valve it closed the connection between the main air pipe and the auxiliary reservoir and opened a feed from the auxiliary reservoir to a brake cylinder which in turn acted on mechanical linkages that operated brake shoes on each wheel. The strength of the brake application depended on the pressure differential between the main air pipe and the train brake pipe - the greater the differential the harder the application.

When the driver moved the brake handle to the release position, re-charging the train brake pipe with compressed air, the triple valve on each vehicle closed the feed from the auxiliary reservoir to the brake cylinder, vented the brake cylinder to the atmosphere and opened the feed from the main air pipe to the auxiliary cylinder.