A system of pipes, valves, air reservoirs, linkages and brake cylinders that control the brakes on a car or train.
The air brakes commonly used by N American railroads were invented by George Westinghouse
The system is charged to the correct pressure through the train line connected to each car, which fills a reservoir on each car. When the engineer reduces pressure in the train line, the difference in pressure between the train line (lower) and the reservoir (higher), causes the brake valve on the car to feed air from the reservoir to the brake cylinder until the pressure between them equalizes. When the engineer releases the brakes by increasing the pressure in the train line, the difference in pressure between the train line (higher) and the reservoir (lower) causes the brake valve on the car to release pressure from the brake cylinder, and charges the reservoir. When there is a large, rapid, release of pressure in the train line, the brake valve will vent the auxiliary/emergency reservoir into the brake cylinder. This is called putting the train in “emergency”. The rapid loss of pressure can be caused by the crew intentionally placing a brake valve in the emergency position (“big holing the train”), by a train line breaking, air hoses coming uncoupled, or by a crew member opening a trainline valve too quickly.