Air Brake System Overview

Trains are heavy. In order to stop or slow down, they need a lot of braking force. Rather than have one vehicle in a train be equipped with brakes, usually all the vehicles are brake-equipped. When a train is braking, each vehicle in it is braking relatively simultaneously. This is accomplished by a compressed air mechanical brake system that runs throughout the train, simply called the train brake .

Each rail vehicle has an air line hose on either end. When vehicles are coupled together, their air lines are connected too. By controlling air pressure with a single control device in the leading vehicle, the entire train's brakes are controlled. The brake system pressure is created by compressors , which can be found on motorized vehicles . Before going into detail about how the system works, let's get familiar with a few of the primary components:

Main Reservoir

Main reservoir is a high volume vessel found on motorized vehicles that are equipped with a compressor . In most cases it is automatically kept pressurized at around 8 bar . The main reservoir is there to provide pressure to the rest of the system.

Brake Pipe

Brake pipe is a system of air lines spanning through a train. On a properly coupled train, air lines are connected between individual vehicles with their valves open. At the ends of the train, however, the air line valves are closed. This allows the pipe to be pressurized at 5 bar in normal running conditions, by the main reservoir. The volume of a brake pipe 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 to be further fed to apply brakes, on demand. While auxiliary reservoirs can take a long time to charge, they are almost impossible to 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.

Train Charging

Due to leaks , no brake system 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 can do 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, as is revving the engine .

Train Brake Application

Train brake is applied by a control device found in motorized rail vehicles. Doing so on a fully charged train functions by air getting dumped from the brake pipe to the atmosphere. This forces auxiliary reservoirs to feed their stored pressure to brake cylinders, pressing brake shoes against wheels.

Train Brake Release

Train brake is released by the same control device. Doing so on a fully charged train functions by the brake pipe getting pressurized by main reservoir. This forces auxiliary reservoirs to recharge from the pipe and no longer feed the brake cylinders, depressing the brake shoes from the wheels.

Automatic Stop Safety Mechanism

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