Air Brake System Overview

Each rail vehicle is equipped with mechanical brakes that function by pressing brake shoes against the wheels. The shoes can be pressed manually with handbrakes , or automatically with compressed air.

Applying brakes with compressed air is quicker, which is why some vehicles are equipped with independent brakes , rather than just handbrakes. Even more importantly, compressed air allows brakes to be applied synchronously throughout an entire train, with the so called train brake . The primary components that make the compressed air brake system are:

Main Reservoir

Main reservoir is a high volume vessel found on motorized vehicles . This reservoir contains air, which is usually automatically pumped up to 8 bar by an onboard compressor . The main reservoir is there to provide pressure to the rest of the brake system.

Brake Pipe

Brake pipe is a system of air lines designed to distribute compressed air throughout an entire train. On a properly coupled train, air line hoses are connected between individual vehicles with their valves open. At the ends of the train, however, the valves are closed. This allows the pipe to be pressurized at 5 bar in normal running conditions, throughout the train, by the main reservoir. The volume of brake pipes is relatively low, but it grows bigger with each additional coupled vehicle.

Auxiliary Reservoirs

Auxiliary reservoirs are medium volume vessels found on each individual vehicle. Pressurized by the brake pipe, they store pressure that is ready to apply brakes on demand. While auxiliary reservoirs can take a long time to charge, they practically never run out during regular operation.

Brake Cylinders

Brake cylinders are low volume vessels found on each individual vehicle, doing the actual braking force application. When pressurized by auxiliary reservoirs, cylinders press brake shoes against wheels, causing vehicles to slow down.

Brake cylinder pressure can be manually dumped . This is useful in situations where brakes need to be released, but waiting for a motorized vehicle to pressurize the system first is undesirable.

Brake Control Valve

Compressed air brakes, be it independent or train , are operated by control devices found in motorized vehicles' cabs . There are different types of control valves and they need to be properly cut in in order to function.

Train Charging

Due to leaks , no brake components can remain pressurized indefinitely. It usually takes some time to bring the components of unused vehicles to their nominal, high pressure level, before they can be set in motion. The two biggest factors are the main reservoir, which will charge as quickly as the respective compressor is capable to, and auxiliary reservoirs, which may take a long time to charge, depending on how many vehicles there are in the train.

Adding additional locomotives to the train to improve charging speeds is a viable option, as is revving the engine .

Automatic Stop Safety Mechanism

Crucial safety feature of the compressed air brake system is that, in case a vehicle connection is severed, such as due to derailment , brake pipe pressure will be lost to the atmosphere, resulting in automatic full brake application on both remaining train parts. This is integral to the compressed air brake system design in trains.