
Railway brake

A railway brake is a type of brake used on the cars of railway trains to enable deceleration, control acceleration (downhill) or to keep them immobile when parked. While the basic principle is similar to that on road vehicle usage, operational features are more complex because of the need to control multiple linked carriages and to be effective on vehicles left without a prime mover. Clasp brakes are one type of brakes historically used on trains.
Early days.
In the earliest days of railways, braking technology was primitive. The first trains had brakes operative on the locomotive tender and on vehicles in the train, where "porters" or, in the United States brakemen, travelling for the purpose on those vehicles operated the brakes. Some railways fitted a special deep-noted brake whistle to locomotives to indicate to the porters the necessity to apply the brakes. All the brakes at this stage of development were applied by operation of a screw and linkage to brake blocks applied to wheel treads, and these brakes could be used when vehicles were parked. In the earliest times, the porters travelled in crude shelters outside the vehicles, but "assistant guards" who travelled inside passenger vehicles, and who had access to a brake wheel at their posts, supplanted them. The braking effort achievable was limited and it was also unreliable, as the application of brakes by guards depended upon their hearing and responding quickly to a whistle for brakes.
An early development was the application of a steam brake to locomotives, where boiler pressure could be applied to brake blocks on the locomotive wheels. As train speeds increased, it became essential to provide some more powerful braking system capable of instant application and release by the train operator, described as a "continuous" brake because it would be effective continuously along the length of the train.
In the United Kingdom, the Abbots Ripton rail accident in January 1876 was aggravated by the long stopping distances of express trains without continuous brakes, which – it became clear – in adverse conditions could considerably exceed those assumed when positioning signals. This had become apparent from the trials on railway brakes carried out at Newark in the previous year, to assist a Royal Commission then considering railway accidents. In the words of a contemporary railway official, these showed that under normal conditions it required a distance of 800 to 1200 yards to bring a train to rest when travelling at 45½ to 48½ mph, this being much below the ordinary travelling speed of the fastest express trains. Railway officials were not prepared for this result and the necessity for a great deal more brake power was at once admitted 
Trials conducted after Abbots Ripton reported the following (for an express train roughly matching one of those involved, like it on a 1 in 200 fall, but unlike it braking under favorable conditions)
However, there was no clear technical solution to the problem, because of the necessity of achieving a reasonably uniform rate of braking effort throughout a train, and because of the necessity to add and remove vehicles from the train at frequent points on the journey. (At these dates, unit trains were a rarity).
The chief types of solution were:
Note: there are a number of variants and developments of all these systems.
The Newark trials showed the braking performance of the Westinghouse air-brakes to be distinctly superior: but for other reasons it was the vacuum system that was generally adopted on UK railways.
Later British practice.
In British practice, only passenger trains were fitted with continuous brakes until about 1930; goods and mineral trains ran at slower speed and relied on the brake force from the locomotive and tender and the brake van—a heavy vehicle provided at the rear of the train and occupied by a guard.
Goods and mineral vehicles had hand brakes which were applied by a hand lever operated by staff on the ground. These hand brakes were used where necessary when vehicles were parked but also when trains were descending a steep gradient. The train stopped at the top of the gradient, and the guard walked forward to "pin down" the handles of the brakes, so the brakes were partially applied during the descent. Early goods vehicles had brake handles on one side only but, from about 1930, brake handles were required on both sides of good vehicles. Trains containing hand-braked vehicles were described as "unfitted": they were in use in Britain until about 1985. From about 1930, semi-fitted trains were introduced, in which goods vehicles fitted with continuous brakes were marshalled next to the locomotive, giving sufficient braking power to run at higher speeds than unfitted trains. A trial in January 1952 saw a 52-wagon, 850 ton, coal train run at an average of , compared to the usual maximum speed on the Midland main line of for unfitted freight trains. In 1952, 14% of open wagons, 55% of covered wagons and 80% of cattle trucks had vacuum brakes.
In the early days of diesel locomotives, a purpose-built brake tender was attached to the locomotive to increase braking effort when hauling unfitted trains. The brake tender was low, so that the driver could still see the line and signals ahead if the brake tender was propelled (pushed) ahead of the locomotive, which was often the case.
By 1878 there were over 105 patents in various countries for braking systems, most of which were not widely adopted.
Continuous brakes.
As train loads, gradients and speeds increased, braking became a problem. In the late 19th century, significantly better "continuous brakes" started to appear. The earliest type of continuous brake was the "chain brake" 
which used a chain, running the length of the train, to operate brakes on all vehicles simultaneously.
The chain brake was soon superseded by air operated or vacuum operated brakes. These brakes used hoses connecting all the wagons of a train, so the operator could apply or release the brakes with a single valve in the locomotive.
These continuous brakes can be simple or automatic, the essential 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 largely "fail safe", though faulty closure of hose taps can lead to accidents such as the Gare de Lyon accident.
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.
Types.
Air versus vacuum brakes.
In the early part of the 20th century, many British railways employed vacuum brakes rather than the railway air brakes used in 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 can be made much more effective than vacuum brakes for a given size of brake cylinder. An air brake compressor is usually capable of generating a pressure of vs only for vacuum. With a vacuum system, the maximum pressure differential is atmospheric pressure ( at sea level, less at altitude). Therefore, an air brake system can use a much smaller brake cylinder than a vacuum system to generate the same braking force. This advantage of air brakes increases at high altitude, e.g. Peru and Switzerland where today vacuum brakes are used by secondary railways. The much higher effectiveness of air brakes and the demise of the steam locomotive have seen the air brake become ubiquitous; however, vacuum braking is still in use in India, Argentina and South Africa, but this will be declining in near future. See "Jane's World Railways".
Air brake enhancements.
One enhancement of the automatic air brake is to have a second air hose (the main reservoir or main line) along the train to recharge the air reservoirs on each wagon. This air pressure can also be used to operate loading and unloading doors on wheat wagons and coal and ballast wagons. On passenger coaches, the main reservoir pipe is also used to supply air to operate doors and air suspension.
Electropneumatic brakes.
The higher performing EP brake uses a "main reservoir pipe" feeding air to all the brake reservoirs on the train, with the brake valves controlled electrically with a three-wire control circuit. This provides between four and seven braking levels, depending on the class of train. It also allows for faster brake application, as the electrical control signal is propagated effectively instantly to all vehicles in the train, whereas the change in air pressure which activates the brakes in a conventional system can take several seconds or tens of seconds to propagate fully to the rear of the train. This system is not however used on freight trains due to cost. 
The system adopted across British Railways from 1950 onwards is described at Electro-pneumatic brake system on British railway trains
Electronically controlled pneumatic brakes.
Electronically controlled pneumatic brakes (ECP) are a development of the late 20th Century to deal with very long and heavy freight trains, and are a development of the EP brake with even higher level of control. In addition, information about the operation of the brakes on each wagon is returned to the driver's control panel.
With ECP, a power and control line is installed from wagon to wagon from the front of the train to the rear. Electrical control signals are propagated effectively instantaneously, as opposed to changes in air pressure which propagate at a rather slow speed limited in practice by the resistance to air flow of the pipework, so that the brakes on all wagons can be applied simultaneously, or even from rear to front rather than from front to rear. This prevents wagons at the rear "shoving" wagons at the front, and results in reduced stopping distance and less equipment wear.
There are two brands of ECP brakes available in North America, one by New York Air Brake and the other by Wabtec. These two types are interchangeable.
Identification.
Air brakes work off high pressure, and the air hoses at the ends of rolling stock are of a small diameter.
On the other hand, vacuum brakes work off low pressure, and the hoses at the ends of rolling stock are of a larger diameter.
Air brakes at the outermost vehicles of a train are turned off using a tap.
Vacuum brakes at the outermost vehicles of a train are sealed by plugs which are sucked into place.
Reversibility.
Brake connections between wagons may be simplified if wagons always point the same way. An exception would be made for locomotives which are often turned on turntables or triangles.
On the new Fortescue railway opened in 2008, wagons are operated in sets, although their direction changes at the balloon loop at the port. The ECP connections are on one side only and are unidirectional.
Accidents with brakes.
Defective or improperly-applied brakes may lead to a runaway train; in some instances this has caused train wrecks: