Air Brake System Overview

Every rail vehicle is equipped with mechanical brakes . They function by physically 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. 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 for the brake system, but sometimes also other systems, such as horns and wipers .

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 available main reservoirs. 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 compressed air that is ready to apply brakes on demand. While auxiliary reservoirs can take a long time to charge, depending on the amount of vehicles in a train, 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 in situations where that may be desired.

Brake Control Valve

Compressed air brakes, be it independent or train , are operated by control valves found in motorized vehicles' cabs , usually in the form of levers. 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 reservoirs, which will charge as quickly as the respective compressor(s) allow it, 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, and so 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 a derailment , emergency brakes are automatically applied on both remaining train parts. This is because the brake pipe pressure is lost to the atmosphere, and it is integral to the compressed air brake system design in trains.