Overview

In the air brake's simplest form, called the straight air system, compressed air pushes on a piston in a cylinder. The piston is connected through mechanical linkage to brake shoes that can rub on the train wheels, using the resulting friction to slow the train. The mechanical linkage can become quite elaborate, as it evenly distributes force from one pressurized air cylinder to 8 or 12 wheels.

The pressurized air comes from an air compressor in the locomotive and is sent from car to car by a train line made up of pipes beneath each car and hoses between cars. The principal problem with the straight air braking system is that any separation between hoses and pipes causes loss of air pressure and hence the loss of the force applying the brakes. This deficiency could easily cause a runaway train. Straight air brakes are still used on locomotives, although as a dual circuit system, usually with each bogie (truck) having its own circuit.

In order to design a system without the shortcomings of the straight air system, Westinghouse invented a system wherein each piece of railroad rolling stock was equipped with an air reservoir and a triple valve, also known as a control valve.
* If the pressure in the train line is lower than that of the reservoir, the brake cylinder exhaust portal is closed and air from the car's reservoir is fed into the brake cylinder to apply the brakes. This action continues until equilibrium between the brake pipe pressure and reservoir pressure is achieved. At that point, the airflow from the reservoir to the brake cylinder is lapped off and the cylinder is maintained at a constant pressure.

* If the pressure in the train line is higher than that of the reservoir, the triple valve connects the train line to the reservoir feed, causing the air pressure in the reservoir to increase. The triple valve also causes the brake cylinder to be exhausted to atmosphere, releasing the brakes.

* As the pressure in the train line and that of the reservoir equalize, the triple valve closes, causing the air pressure in the reservoir and brake cylinder to be maintained at the current level.

Unlike the straight air system, the Westinghouse system uses a reduction in air pressure in the train line to apply the brakes. When the engineer (driver) applies the brake by operating the locomotive brake valve, the train line vents to atmosphere at a controlled rate, reducing the train line pressure and in turn triggering the triple valve on each car to feed air into its brake cylinder. When the engineer releases the brake, the locomotive brake valve portal to atmosphere is closed, allowing the train line to be recharged by the compressor of the locomotive. The subsequent increase of train line pressure causes the triple valves on each car to discharge the contents of the brake cylinder to atmosphere, releasing the brakes and recharging the reservoirs.

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.

Modern air brake systems are in effect two braking systems combined:

* The service brake system, which applies and releases the brakes during normal operations, and
* The emergency brake system, which applies the brakes rapidly in the event of a brake pipe failure or an emergency application by the engineer.

When the train brakes are applied during normal operations, the engineer makes a "service application" or a "service rate reduction”, which means that the train line pressure reduces at a controlled rate. It takes several seconds for the train line pressure to reduce and consequently takes several seconds for the brakes to apply throughout the train. In the event the train needs to make an emergency stop, the engineer can make an "emergency application," which immediately and rapidly vents all of the train line pressure to atmosphere, resulting in a rapid application of the train's brakes. An emergency application also results when the train line comes apart or otherwise fails, as all air will also be immediately vented to atmosphere.

In addition, an emergency application brings in an additional component of each car's air brake system: the emergency portion. The triple valve is divided into two portions: the service portion, which contains the mechanism used during brake applications made during service reductions, and the emergency portion, which senses the immediate, rapid release of train line pressure. In addition, each car's air brake reservoir is divided into two portions—the service portion and the emergency portion—and is known as the "dual-compartment reservoir”. Normal service applications transfer air pressure from the service portion to the brake cylinder, while emergency applications cause the triple valve to direct all air in both the service portion and the emergency portion of the dual-compartment reservoir to the brake cylinder, resulting in a 20–30% stronger application.

The emergency portion of each triple valve is activated by the extremely rapid rate of reduction of train line pressure. Due to the length of trains and the small diameter of the train line, the rate of reduction is high near the front of the train (in the case of an engineer-initiated emergency application) or near the break in the train line (in the case of the train line coming apart). Farther away from the source of the emergency application, the rate of reduction can be reduced to the point where triple valves will not detect the application as an emergency reduction. To prevent this, each triple valve's emergency portion contains an auxiliary vent port, which, when activated by an emergency application, also locally vents the train line's pressure directly to atmosphere. This serves to propagate the emergency application rapidly along the entire length of the train.