Why can a drop in a train’s brake-pipe pressure apply its brakes?

A conventional train air-brake system includes a pipe running along the train and an air reservoir on each vehicle. The pipe does more than deliver air: changes in its pressure also communicate a braking command.

When the driver reduces brake-pipe pressure, a control valve on each vehicle responds to the difference between the pipe and stored reservoir pressure. The valve sends local reservoir air into a brake cylinder.

The cylinder pushes a mechanical linkage that applies friction brakes. Energy to apply them comes from compressed air already stored near the wheels. The pressure change in the long pipe tells the distributed equipment when to act.

A separated pipe can also cause a pressure drop, giving the arrangement an important protective response. It still depends on charged reservoirs and functioning components. Control through falling pressure therefore combines communication and stored energy rather than assuming that losing any air always means losing all braking.