Railway brake
Brakes are the devices 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 engine.
- putting the engine 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 engine to increase braking effort.
- engineering the lines with gentle gradients so that weak brakes were reasonably effective.
- catchpoints and safety siding 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, etc, braking became a problem. After 40 years of railways, significantly better continuous brakes started to appear, either air operated or vacuum operated. These brakes used an air hose connecting all the wagons of a train, so that the engine driver could apply or release the brakes with a single valve on the engine.
These continuous brakes could 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 spring, which applies the brakes if pressure is lost. Automatic brakes are thus to an extent "fail safe".
The standard Westinghouse Air Brake has the additional enhancement of the triple valve, and local reserviors 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 air is void 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
The biggest advantage that air brakes have over vacuum brakes is that the maximum pressure is 70psi instead of only 17psi. Air brake equipment can thus also be smaller and lighter. This advantage increases at high altitude, e.g. Peru. Operation is also faster, useful on suburban lines with many stops.
The main advantage for vacuum brakes is that the vacuum can be created by a steam ejector with no moving parts, whereas an air brake system requires a noisy pump.
By and large, the vacuum brake has lost the battle for new systems, and many vacuum braked systems change to air brakes when the demand for braking power increases.
Air brake enhancements
One enhancement of the automatic air brake is to have a second air hose along the train to recharge the air reservoirs on each wagons. This air pressure can also be used to operated loading and unloading doors on wheat, coal and ballast wagons.