How do train air brakes work?

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How do train air brakes work?

Freight trains depend on air for their brake applications in two ways:

To signal the brakes to apply or release by changing air pressure through a Brake Pipe that runs the length of the train.
To apply the air to the brake cylinder from a reservoir (called an auxiliary reservoir) on each wagon.

The key to this system is a set of valves on the wagons called a triple valve or control valve.

This set of valves has three functions:

To allow air to flow from the wagon's auxiliary reservoir to the brake cylinders when the brake pipe pressure drops.
To release air from the brake cylinder when the brake pipe pressure rises to its full pressure (500 kPa in Australia).
To recharge the wagon's reservoir after the brakes are released and the brake pipe pressure rises to 500 kPa.

How does the air brake system function? When brake pipe pressure drops, the reduced air pressure opens the application valve in the wagon's triple valve. This opens a port, allowing air to flow to the brake cylinder and apply the brakes to the wheels.

When the air pressure in the brake pipe rises, the increase in pressure;

closes the application valve so no more air flows to the brake cylinders,
opens a release valve that releases the air from the brake cylinders, and
opens a charging valve that allows air to flow from the brake pipe into the auxiliary reservoir, readying it for the next brake application.

Because of the system's design, the reservoirs cannot be recharged while a brake application is active. The brakes must be fully released, the reservoirs recharged, and then reapplied.

Enhancements: The relayed or twin pipe system

To overcome some of these problems, a second pipe is often fed along the wagon from the main locomotive reservoirs at around 800 kPa (average locomotive main reservoir pressure). This continually charges a second supplementary reservoir on the wagons.

This is called a relayed or two-pipe system.

While effective, this pressure difference complicates brake activation. The triple valve and brake cylinders are set up for 500 kPa, and the triple valve still needs to control the brake applications.

A second relay valve is now needed to connect the higher-pressure supplementary air to the brake cylinders.

To achieve this, the triple valve feeds air to a dummy brake cylinder or dummy volume. This air pressure then activates the relay valve to open and allow air from the supplementary reservoir to flow to the brake cylinders at the required pressure.

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How do train air brakes work?

How do train air brakes work?

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