On railways an air brake is a brake operated by compressed air. A safer air brake was patented by George Westinghouse on March 5, 1872. Westinghouse's invention revolutionized the railroad industry, making stopping reliable and thus permitting trains to travel at higher speeds. Westinghouse made many alterations to improve his invention leading to various forms of the automatic brake. The United States Congress made their use mandatory by the US Railroad Safety Appliance Act. By 1905, over 2,000,000 freight, passenger, mail, baggage and express railroad cars and 89,000 locomotives were equipped with the Westinghouse Quick-Action Automatic Brake.

In the air brake's simplest form, compressed air pushes on a piston in a cylinder. The piston is connected to a brake shoe which can rub on the train wheel, creating friction and stopping the train. The compressed air comes from an air compressor in the locomotive and is sent from car to car by a string of hoses and pipes. There are three problems with this approach:

• it takes a long time to get enough air to the end of the train, so the brakes apply too slowly
• if there is any opening in the string of hoses and pipes, air will leak out and the brakes will not work.
• In particular, if the coupler between cars comes apart, the rear section will have no brakes at all (locomotives have their own brakes) potentially causing a runaway train.

Westinghouse invented a triple valve that was installed on each railroad car. It allowed compressed air from the locomotive to fill a reservoir tank on each car. As long as there is enough air pressure in the feed line through the train, the triple valve in each car keeps the brakes off and the reservoir charged. When pressure in the train line drops, the triple valve sends air from the car's reservoir to the car's brake cylinder, applying the brakes. This system is fail safe, meaning that any failure in the feed line, including a separation of part of the train, will cause the brakes to be applied, stopping the train.

Other applications

Air brakes are also used in trucks, buses and semi-trailers.

The air brakes on a semi-trailer are connected to a tractor with two lines. One line is called the supply line or the emergency line. It is usually larger and is red or has red fittings. The ignatz line provides air pressure to fill the semi-trailer's reservoir tank and the pistons that activate the brakes. The other line is called the service line. It is usually smaller and is blue or has blue fittings.

In normal braking pressing the brake pedal pressurizes the service line. This activates a valve in the trailer which directs air from the reservoir and the emergency line to the brake cylinders where it moves the piston that activates the brakes. When the pedal is let up the service line pressure is decreased. When the service line pressure drops it causes the valve in the trailer to block the air supply from the reservoir while releasing the pressure in the brake cylinder and the brakes are released. The system is a form of servo or amplifier.

If the pressure in the emergency line drops, due to the activation of a valve in the cab, the disconnection of the emergency line coupling or a break in the emergency line, a check valve prevents air from escaping the reservoir and air pressure from the reservoir activates the brakes.

When a semi-trailer is disconnected from the tractor, pressure from the reservoir applies the emergency brakes because the emergency line is disconnected. Eventually air will leak out of the system and there will no longer be air pressure to apply the emergency brakes. Newer trailers are also equiped with spring brakes. When there is air pressure in the reservoir it supplies air to a piston which counteracts the spring. When the air pressure in the reservoir drops, the spring brake cylinder can no longer counteract the spring and the brakes are applied.

External reference

• the Railway-Technical Air Brakes page