train-braking
Trains can be incredibly long. Imagine a vehicle so long that a section of it could be ascending while another portion could be descending.
Stopping a moving train has been a perennial problem with various solutions throughout history. By late 1800s, there were at least a 100 patents for railway braking systems. The distance a train travels while decelerating from a given speed to rest is known as its braking distance at that speed. A large braking distance can prove hazardous in case of emergencies, potentially defeating the purpose of brakes. Braking is thus a direct limiter on how fast a train can go, even in the presence of very capable locomotives and strong, load-bearing tracks.
A North Eastern Railway document from 1877 suggests 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”
An early solution was to simply apply enough brake power on the locomotive to bring the vehicle to the stop. While early technology involved screws and linkage activating brake blocks in the tender car, it was soon developed to be actuated using steam diverted from the locomotive.
As train speeds and loads increased, however, the braking force from the locomotive alone was insufficient, and brakes were installed in individual cars. It was the job of brake men to walk the length of the train, and apply brakes in each car at the behest of the locopilot. There were many points of failure to this system, starting from non-uniform braking forces across the train to even bigger concerns such the brake man missing signal from the locomotive to start braking.
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Earliest form of continuous braking was using chains, which ran through the length of the train. This allowed brakes on all cars to be activated at once. Although this system was pretty limited in terms of the length of the train it was capable of handling - frequently not more than 3-5 cars. An unexpected challenge was to account for the sag in pin-couplers while installing these brakes throughout the bottom of trains.
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Heberlein brake, designed independently in Germany was a comparitively nifty option involving overhead cables controlling brake clips on the wheels in each wagon. While they typically run over the train, the cables are also occasionally connected underneath the train. Heberlein brakes also had a fail safe mechanism that would automatically engage brakes if the cable snapped at any point. In fact, it is still legal to use this type of brake in Germany with some gauge and speed restrictions.
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In British railway networks, continuous braking was used only for passenger trains while goods trains continued to rely on braking from the locomotive and a car the end of the train called a R:brake-vans. These continued to run at slower speeds.
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In addition, handbrakes were attached to each car, which can be operated by someone on the ground. These were used to park trains as well as control speeds while the train passed a downgrade. Continuous braking in goods trains was introduced much gradually, with new “fitted” cars marshalled next to the locomotive.
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Between the 60s and 80s, with the advent of diesel engines (which were lighter than Steam locomotives and hence had high tractive but low braking power), British trains also had additional brake-tenders. These were a skeumorph of coal/water tenders attached to steam engine, and were designed to be low and heavy wagons (typically weighing around 36 tons) attached to the front of a locomotive and supplementing braking force. These were decomissioned as more trains came “fitted” with continuous braking.
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Modern continuous braking involves air or vacuum-operated brakes. These systems used hoses connecting all the wagons in the train, allowing brake actuation to be controlled using a single valve. Unlike the Heberline brakes, early air-controlled continous brakes did not engage should there be a disconnection in the connecting hose pipe. Newer automatic brakes use a similar system, but operate with negative pressure state where the brake hoses contained vacuum and needed pressure for braking. This way, snapping of hoses at any point led exposure to atmospheric pressure that in turn activated the brakes.
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Of course, this doesn’t come without its challenges. Upon brake application, as air travels along the length, brakes in cars at the head of the train engage earlier than the ones further behind, resulting in longitudinal forces (assuming the train is on a straight line) on the train. This can also get complicated if the train is on a curve.
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Automatic vacuum brakes are canonical in most trains at this point. In passenger trains, they have the added benefit of being exposed as emergency brakes that can be triggered by passengers within cars,
Brake systems on trains has a long history. While steel wheels on steel tracks significantly increase a train’s capacity to transport loads, it also limits the forces available and so velocity changes occur in trains relatively slowly. Brake systems are in a constant state of development and continue to limit trains from operating at speeds they are capable at travelling - the Shinkansen Fastech 360 is not feasible for operation at rated speed because of a ridiculously large 7000m emergency braking distance. Nevertheless, trains run thousands of miles daily with very few incidents.