Air Brake System

The air brake system is the pneumatic braking system carried on each freight car and locomotive in a train. It uses compressed air, controlled through a train-line pipe, to apply and release the brakes on every vehicle in the consist at once.

Main Parts

A car's air brake equipment consists of the brake pipe that runs the length of the train, an air brake control valve, one or more air reservoirs, an air brake cylinder, and the mechanical brake rigging that ends at the brake shoes. The brake pipe is coupled between cars so that air and brake commands pass from the locomotive through the whole train.

How It Works

The standard AAR freight brake is an automatic air brake. The locomotive normally keeps the brake pipe charged to a set pressure. When the engineer makes a brake application, the brake pipe pressure is reduced. The control valve on each car senses that reduction and lets air stored in the car reservoirs flow into the brake cylinder. The cylinder pushes the brake rigging, forcing the shoes against the wheel treads. Releasing the brakes means recharging the brake pipe; the control valve then vents the cylinder and lets the reservoirs refill.

Fail-Safe Design

Because a drop in brake pipe pressure is what applies the brakes, the system is fail-safe: if the train pulls apart or a hose bursts, the brake pipe loses pressure and the brakes apply automatically. Air brake equipment, testing, and condemnable conditions are governed by the AAR rules and field manual.