[P.O.E.]

Under the Westinghouse system, therefore, brakes are applied by reducing train line pressure and released by increasing train line pressure. The Westinghouse system is thus fail safe—any failure in the train line, including a separation ("break-in-two") of the train, will cause a loss of train line pressure, causing the brakes to be applied and bringing the train to a stop, thus preventing a runaway train.

http://en.wikipedia.org/wiki/Railway_air_brake

[thackney]

But that system is not intended to hold the train indefinitely. It is designed to give the train operators time to set each hand brake on each train car.

[jim_trent]

That is similar to the way that over-the-road, 18-wheelers work. Lose air pressure and the brakes go on. You need air pressure to release the brakes. Springs lock the brakes if there is not enough continuous pressure to keep them unlocked.