Brake (railway)

Brakes are used on railway trains to bring the train to a standstill.

In the earliest days of railways, braking was primitive and relied on a mixture of:

• brakes on the locomotive.
• putting the locomotive into reverse.
• calling for additional application of handbrakes by whistle signals.
• hauling special brake vans to assist with the braking effort.
• pushing special brake tenders ahead of a diesel locomotive to increase braking effort.
• engineering the lines with gentle gradients so that weak brakes were reasonably effective.
• catchpoints and safety sidings to prevent runaways doing too much damage.
• sidings to enable heavy trains to be divided into convenient portions.
• applying and releasing handbrakes at the top and bottom of steep descents.

As train loads increased, gradients steepened, and speeds increased, braking became a problem. In the late 19th century, significantly better continuous brakes started to appear, either air operated or vacuum operated. These brakes used hoses connecting all the wagons of a train, so the driver could apply or release the brakes with a single valve in the locomotive.

These continuous brakes can be simple or automatic, the key difference being what happens should the train break in two. With simple brakes, pressure is needed to apply the brakes, and all braking power is lost if the continuous hose is broken for any reason. Simple non-automatic brakes are thus useless when things really go wrong, as is shown with the Armagh rail disaster.

Automatic brakes on the other hand use the air or vacuum pressure to hold the brakes off against a reservoir carried on each vehicle, which applies the brakes if pressure/vacuum is lost in the train pipe. Automatic brakes are thus to a great extent "fail safe".

The standard Westinghouse Air Brake has the additional enhancement of a triple valve, and local reservoirs on each wagon that enable the brakes to be applied fully with only a slight reduction in air pressure, reducing the time that it takes to release the brakes as not all pressure is voided to the atmosphere.

Non-automatic brakes still have a role on engines and first few wagons, as they can be used to control the whole train without having to apply the automatic brakes.

Air versus vacuum brakes

In the early part of the 20th century, many British railways employed vacuum brakes rather than the air brakes used in America and much of the rest of the world. The main advantage of vacuum was that the vacuum can be created by a steam ejector with no moving parts (and which could be powered by the steam of a steam locomotive), whereas an air brake system requires a noisy and complicated compressor.

However, air brakes are much more efficient than vacuum brakes, capable of generating a pressure of 90psi vs only 15psi for vacuum. With a vacuum system, the maximum pressure differential is atmospheric pressure (14.7psi at sealevel, less at altitude). This advantage of air brakes increases at high altitude, e.g. Peru. This much higher efficiency, and the demise of the steam locomotive, has seen the air brake become ubiquitous; vacuum braking is no longer in large scale use anywhere in the world.

Air brake enhancements

One modern enhancement of the automatic air brake is to have a second air hose (the main reservioir or main line) along the train to recharge the air reservoirs on each wagons. This air pressure can also be used to operate loading and unloading doors on wheat, coal and ballast wagons. On passenger coaches, the main reservoir pipe is also used to supply air to operate the external doors, and where fitted powered internal doors and air suspension.

References

• British Transport Commission, London (1957:142). Handbook for Railway Steam Locomotive Enginemen.

See also

• Electromagnetic brake
• Regenerative brake
• Emergency brake
• Air brake (rail)
• vacuum brake