Railway brake

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 conditions involved (such as a 1 in 200 downward run, but not braking under favorable conditions): • The chain brake, in which a chain was connected continuously along the bottom of the train. When pulled tight, it activated a friction clutch that used the rotation of the wheels to tighten a brake system at that point; this system has severe limitations in length of train capable of being handled (as braking strength was considerably weaker after the third car), and of achieving good adjustment (give the slack that pin couplers required, which a fixed-length chain could not account for). In the United States, the chain brake was independently developed and patented by Lucious Stebbins of Hartford, Connecticut in 1848 and by William Loughridge of Weverton, Maryland in 1855. The British version was known as the Clark and Webb Brake, after John Clark, who developed it throughout the 1840s, and Francis William Webb, who perfected it in 1875. The chain brake remained in use until the 1870s in America • The simple vacuum system. An ejector on the locomotive created a vacuum in a continuous pipe along the train, allowing the external air pressure to operate brake cylinders on every vehicle. This system was very cheap and effective, but it had the major weakness that it became inoperative if the train became divided or if the train pipe was ruptured. • The automatic vacuum brake. This system was similar to the simple vacuum system, except that the creation of vacuum in the train pipe exhausted vacuum reservoirs on every vehicle and released the brakes. If the driver applied the brake, his driver's brake valve admitted atmospheric air to the train pipe, and this atmospheric pressure applied the brakes against the vacuum in the vacuum reservoirs. Being an automatic brake, this system applies braking effort if the train becomes divided or if the train pipe is ruptured. Its disadvantage is that the large vacuum reservoirs were required on every vehicle, and their bulk and the rather complex mechanisms were seen as objectionable. • The Westinghouse air brake system. In this system, air reservoirs are provided on every vehicle and the locomotive charges the train pipe with a positive air pressure, which releases the vehicle brakes and charges the air reservoirs on the vehicles. If the driver applies the brakes, his brake valve releases air from the train pipe, and triple valves at each vehicle detect the pressure loss and admit air from the air reservoirs to brake cylinders, applying the brakes. The Westinghouse system uses smaller air reservoirs and brake cylinders than the corresponding vacuum equipment, because a moderately high air pressure can be used. However, an air compressor is required to generate the compressed air and in the earlier days of railways, this required a large reciprocating steam air compressor, and this was regarded by many engineers as highly undesirable. A further drawback was the need to release the brake completely before it could be re-applied—initially there was no "graduable release" available and numerous accidents occurred while the brake power was temporarily unavailable. 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==