The Straight Air Brake
The first form of the air brake consisted of an air pump, a main reservoir, and an engineer's valve on the locomotive, and of a train pipe and brake cylinder on each car. One problem with this first form of the air brake was that braking was applied to the first cars in a train much sooner than to the rear cars, resulting in shocks and damages when the rear cars bunted against the cars ahead of them. The main objection however was that it was not an automatic brake, i.e. even a minor mishap like a broken coupling left the entire train without any brake power at all.
The Plain Automatic Air Brake
In 1872, George Westinghouse invented the automatic air brake by inventing the triple valve and by equipping each car with its own air cylinder. Air pressure is maintained in the auxiliary reservoirs and in the train pipe at all times when the brakes are not applied. An equilibrium of air pressure is maintained in the train pipe and in the auxiliary air cylinders. To apply the brakes to all of the cars at about the same time, pressure is released from the train pipe, causing the triple valve on each car to apply the brakes. To release the brakes on each car, pressure is increased in the train pipe until an excess pressure above that of the pressure in each auxiliary cylinder is reached, which throws the triple valve so as to close the inlet to the brake cylinder and open the inlet to the auxiliary reservoir from the train pipe, thus allowing the equilibrium of the two pressures to be reached.
The Quick Action Triple Valve
Although the plain automatic air brake was a great improvement over the straight air brake, in an emergency the system still applied the brakes to the last cars in a train later than to the first cars in a train. To remedy that condition, George Westinghouse invented the quick action triple valve in 1887. It automatically vents air from the brake pipe locally on each car, which applies the brakes more quickly.