Trains are generally equipped with vacuum brakes. In steam locomotives, ejectors help create the vacuum and in diesel and electric locomotives, air-compressors and vacuum exhausters are used.
Pipes passing through the underside of coaches are connected to vacuum cylinders (provided in the coach) which help maintain a vacuum throughout the length of the pipe. The pipes (of each coach) are then linked.
When the train is on the run, a vacuum of about 20-22 inches per square inch is maintained throughout the system (atmospheric pressure is 30 pounds per square inch).
When the driver wants to stop the train, he gradually allows air to enter the vacuum pipe and activates the pistons of the vacuum cylinders which help to apply the brake.
For emergency application of brakes by passengers, coaches are provided with alarm chains connected to the vacuum system. When the chain is pulled, a small valve open up and allows air to enter the system slowly, gradual application of the brakes.
Direct admission valves are provided in each coach near the vacuum cylinder and they help in the uniform application of the brake throughout the train.