Air Brake System Overview

This is an old version of the manual page from B98, remove this warning after verifying it's up to date

In order for trains to be able to slow down effectively, each containing vehicle needs to apply brakes relatively simultaneously. This is accomplished by the train’s air brake system. Each rail vehicle comes equipped with an air line hose on either end. When vehicles are coupled together, their air lines are connected too, in a handshake manner. This way, the driver in a leading vehicle is able to operate brakes on the entirety of the train, using a single train brake control device.

The air in the brake system is pumped by compressors found onboard the motorized vehicles, and it is shared throughout the train via a system of valves, pipes and hoses. For simplicity, the system can be viewed as composed of three separate units: main reservoir, brake pipe and brake cylinder.

Main reservoir is a high volume vessel found onboard motorized vehicles. It is usually kept pressurized to a high level by the onboard compressor, and serves to provide pressure to the rest of the system.

Brake pipe, pressurized by the main reservoir, is a system of valves, pipes and hoses stretched throughout a whole train. At every coupling, the airflow can be manually opened or closed, by a valve at its base, called the angle cock. This is done on the ends of the train, to prevent the pressurized air from escaping into the atmosphere. Under normal running conditions the brake pipe holds a pressure of 5 bar.

Finally, every individual vehicle has one or more of its own brake cylinders. These are low volume vessels that put pressure on a piston, which presses the vehicle’s brake shoes against the wheels, causing it to slow down. A control valve reacts to changes in pressure in the brake pipe, pressurizing the brake cylinders with air accordingly, from specialized vessels found on each vehicle, called auxiliary reservoirs.

By operating the train brake lever, the driver controls the brake pipe pressure, indirectly affecting the amount of air supplied to each brake cylinder in the train. By moving the train brake lever in the "apply" direction, the driver dumps air from the brake pipe into the atmosphere. The control valves on each vehicle release pressure contained in the auxiliary reservoirs into the brake cylinders and thus apply the brakes. By moving the train brake lever in the "release" direction, the driver pressurizes the brake pipe with air from the main reservoir. This refills the auxiliary reservoirs and the control valves react by dumping brake cylinder pressure into the atmosphere, releasing the brakes.

In case the connection between the vehicles is broken, full brakes will apply automatically on both remaining train parts. This safety feature is integral to the pressurized air brake system design. Brake pipe needs to be fully pressurized so that the brakes are released. When the brake pipe loses all pressure, brakes get fully applied. This is also one of the disadvantages of the system – before a train can be safely set in motion, it must be pressurized first. The longer the train, the longer it takes to pressurize the system, particularly the auxiliary reservoirs. With long trains, adding extra locomotives can help pressurize the system faster.