In general, this conventional system consists of a brake pipe that runs the length of the train which supplies air to reservoirs mounted on each of the cars.
This method of controlling the brakes on freight and passenger cars has remained virtually unchanged since its invention by George Westinghouse in 1868.
The cable also doubles as a communication medium that allows the locomotive to send commands to, and receive feedback from, the cars along the length of the train.
This provides much better train control, shortens the stopping distances, and leads to a lower risk of derailment or of coupling breakage.
Further, since the cars can also send their status to the locomotive at the front, the engineer can monitor the state of the train and know at any given time the braking capabilities available.
[1] The benefits are better control of braking, less equipment wear from pushing and pulling between cars, shorter stopping distance and improved headways.
This has changed with the introduction of electronic controls, allowing data to be transmitted by two-conductor wire or radio from the locomotive to a microprocessor on each car, where locally powered valves hold the desired pressure in each brake cylinder.
In 2014, the U.S. Federal Railroad Administration proposed that electronic braking be required on trains carrying hazardous materials.