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A coupling (or a coupler) is a mechanism for connecting rolling stock in a train. The design of the coupler is standard, and is almost as important as the track gauge, since flexibility and convenience are maximised if all rolling stock can be coupled together.
The equipment that connects the couplings to the rolling stock is known as the draft gear.
The different types of coupling do not always have formal or official names, which makes descriptions of the couplings in use on any railway system problematic.
The standard type of coupling on railways following the British tradition is the buffer and chain coupling used on the pioneering Planet class locomotive of the Liverpool and Manchester Railway of 1830. These couplings followed earlier tramway practice but were made more regular. The vehicles are coupled by hand using a hook and links with a turnbuckle that draws the vehicles together. In Britain, this is called a screw coupling. Vehicles have buffers, one at each corner on the ends, which are pulled together and compressed by the coupling device. This arrangement limits the slack in trains and lessens shocks. The earliest buffers were fixed extensions of the wagon frames, but later spring buffers were introduced.
The link-and-pin coupling was the original style of coupling used on North American railways. After most railroads converted to semi-automatic Janney couplers, the link-and-pin survived on forestry railways. While simple in principle, the system suffered from a lack of standardisation regarding size and height of the links, and the size and height of the pockets.
The link-and-pin coupler consisted of a tube-like body that received an oblong link. During coupling, a rail worker had to stand between the cars as they came together and guide the link into the coupler pocket. Once the cars were joined, the employee inserted a pin into a hole a few inches from the end of the tube to hold the link in place. This procedure was exceptionally dangerous and many brakemen lost fingers or entire hands when they did not get them out of the way of the coupler pockets in time. Many more were killed as a result of being crushed between cars or dragged under cars that were coupled too quickly. Brakemen were issued with heavy clubs that could be used to hold the link in position, but many brakemen would not use the club, and risked injury.
The link-and-pin coupler proved unsatisfactory because:
An episode of the 1958 TV series Casey Jones was devoted to the problems of link-and-pin couplings.
To avoid the safety issues, Karl Albert, then director at the Krefeld Tramway, developed during the 1920s the Albert coupler, a key and slot coupler with two pins. Cars to be coupled were pushed together, both couplings moving to the same side. One pin was inserted, then the cars were pulled to straighten the coupling and the other pin inserted. This operation required less exact shunting. Due the single piece design, only minimal slack was possible. The system became quite popular with tram systems and narrow gauge lines.
During the 1960s most cities replaced them by automatic couplers. But even in modern cars, Albert couplers get installed as emergency couplers for towing a faulty car.
The link and pin was replaced in North American passenger car usage during the latter part of the 19th century by the assemblage known as the Miller Platform, which included a new coupler called the Miller Hook. The Miller Platform (and hook coupler) was used for several decades before being replaced by the Janney coupler.
Norwegian (or meat chopper) couplings consist of a central buffer with a mechanical hook that drops into a slot in the central buffer. There may also be a U-shaped securing latch on the opposite buffer which is fastened over the top of the hook to secure it. The Norwegian is found only on narrow gauge railways of 1,067 mm (3 ft 6 in), 1,000 mm (3 ft 3 3⁄8 in) or less, such as the Isle of Man Railway, Western Australian Government Railways, the Ffestiniog Railway and the Welsh Highland Railway, Tanzania, where low speeds and reduced train loads allow a simpler system. The Norwegian coupler allows sharper curves than the buffer-and-chain, which is an advantage on those railways.
On railway lines where rolling stock always points the same way, the mechanical hook may be provided only on one end of each wagon. Similarly, the hand brake handles may also be on one side of the wagons only.
Norwegian couplings are not particularly strong, and may be supplemented by auxiliary chains.
Not all Norwegian couplings are compatible with one another as they vary in height, width, and may or may not be limited to one hook at a time.
There are a number of automatic train couplings, most of which are mutually incompatible.
Later Master Car Builders Association coupler, now AAR (Association of American Railroads) coupler; also commonly known as a BuckEye, Knuckle, or Alliance coupler. The AAR/APTA TypeE, TypeF, and TypeH couplers are all compatible Janney couplers, but used for different rail cars (general freight, tank cars, rotary hoppers, passenger, etc.).
The knuckle coupler or Janney coupler was invented by Eli H. Janney, who received a patent in 1873 (U.S. Patent 138,405). It is also known as a "buckeye coupler", notably in the United Kingdom, where some rolling stock (mostly for passenger trains) is fitted with it. Janney was a dry goods clerk and former Confederate Army officer from Alexandria, Virginia, who used his lunch hours to whittle from wood an alternative to the link and pin coupler. The term Buckeye comes from the nickname of the US state of Ohio, the "Buckeye State" and the Ohio Brass Company which originally marketed the coupling.
In 1893, satisfied that an automatic coupler could meet the demands of commercial railroad operations and, at the same time, be manipulated safely, the United States Congress passed the Safety Appliance Act. Its success in promoting switchyard safety was stunning. Between 1877 and 1887, approximately 38% of all railworker accidents involved coupling. That percentage fell as the railroads began to replace link and pin couplers with automatic couplers. By 1902, only two years after the SAA's effective date, coupling accidents constituted only 4% of all employee accidents. Coupler-related accidents dropped from nearly 11,000 in 1892 to just over 2,000 in 1902, even though the number of railroad employees steadily increased during that decade.
When the Janney coupler was chosen to be the North American standard, there were 8,000 patented alternatives to choose from. The only significant disadvantage of using the Janney design is that sometimes the drawheads need to be manually aligned. Many AAR coupler designs exist to accommodate requirements of various car designs, but all are required to have certain dimensions in common which allow for one design to couple to any other.
The Russian SA3 coupler works according to the same principles as the AAR coupler but is incompatible. It was introduced during the rebuilding of the railway network in the Soviet Union after the Second World War and has since been used on the whole broad-gauge network, including Mongolia. It is also used on the standard gauge networks of Iraq and on Malmbanan in Sweden for ore trains.
Unicoupler has been developed by Knorr company from Germany in the 1970s and is widely used in Iran in freight cars. This type of coupler is compatible with SA-3 and Willison couplers. The Unicoupler is also known as AK69e. The Unicoupler was the West-European development, it was developed in parallel with a compatible East-European counterpart, the Intermat coupler.
The C-AKv coupler is a newer compact Willison coupler developed by Faiveley Transport. It is mechanical fully compatible to the SA3 coupler and the Unicoupler and if additional buffers are mounted it can be coupled with the conventional European screw coupling as well.
Multi-function couplers (MFCs) are 'fully automatic' couplers that make all connections between the rail vehicles (mechanical, air brake and electrical) without human intervention, in contrast to autocouplers which just handle the mechanical aspects. The majority of trains fitted with these types of couplers are multiple units, especially those used in mass transit operations.
There are a few designs of fully automatic couplers in use worldwide, including the Scharfenberg coupler, various knuckle hybrids such as the Tightlock, (used in the UK), the Wedgelock coupling, Dellner couplings (similar to Scharfenberg couplers in appearance), BSI coupling (Bergische Stahl Industrie now Faiveley Transport) and the Schaku-Tomlinson Tightlock coupling.
There are a number of other automatic train couplings similar to the Scharfenberg coupler, but not necessarily compatible with it. Older US transit operators continue to use these non-Janney electro-pneumatic coupler designs and have used them for decades.
The Westinghouse H2C coupler is currently used on the R32, R42, R62, R62A, R68, and R68A class subway cars of the New York City Subway. The A ends of the cars typically have the Westinghouse coupler and the B ends use either a semi-permanent drawbar, or a Westinghouse coupler.
The WABCO N-Type coupler was first developed for the prototype Pittsburgh Skybus system with the initial model N-1 as applied only to the three Skybus cars. The updated model N-2 with a larger 4-inch (102 mm) gathering range was first applied to the new "Airporter" rapid transit cars on the Cleveland Rapid Transit line. The model N-2 used lightweight draft gear slung below the center sill, to allow for the wide swings required to go around sharp curves. This made the N-2 unsuitable for main line railroad use so an updated version N-2-A was developed for that market. The first of these were fitted in 1968 to the UAC TurboTrain with 228 electrical contacts and the Budd Metropolitan EMU with 138 contacts. Starting in the 1970s the N-2-A was fitted to the entire SEPTA Silverliner family of MU's, the NJT Arrow series of MU's and the Metro-North/LIRR M series of MU railcars. The N-2 was also used by the PATCO Speedline, but was replaced due to issues with the electrical contacts. Later WABCO would create a new model N-3 for the BART system with a 6-by-4-inch (152 mm × 102 mm) gathering range which required a rectangular funnel.
The WABCO N-type is sometimes referred to as the pin and cup coupler or spear coupler.
The Tomlinson coupler was developed by the Ohio Brass Company for mass transit applications, but eventually found use in some mainline railroad vehicles as well. It consists of two squared metal hooks that engage with each other in a larger rectangular frame with air line connections above and below. Since the coupler's development the manufacturing arm of Ohio Brass was purchased by WABCO which now manufacturers the line along with the N-type. The Tomlinson coupler is the most widely used fully automatic heavy rail coupling in North America having been adopted by the DC Metro, MBTA, PATCO Speedline, SEPTA Broad Street Subway, LA Metro, Baltimore Metro, Miami Metro, MARTA Rail and the New York City Subway for its R44/46 fleet and all modern classes starting with the R142. For applications outside of rapid transit the coupler had to be significantly enlarged to meet the increased strength requirements first appearing in this capacity on the Budd Metroliner and later on the Illinois Central Highliner fleet. Its relative lack of strength is one reason the N-Type has been more successful in the mainline railroad arena.
The Scharfenberg coupler (German: Scharfenbergkupplung or Schaku) is probably the most commonly used type of fully automatic coupling. Designed in 1903 by Karl Scharfenberg in Königsberg, Germany (today Kaliningrad, Russia), it has gradually spread from transit trains to regular passenger service trains, although outside Europe its use is generally restricted to mass transit systems. The Schaku coupler is superior in many ways to the AAR (Janney/Knuckle) coupler because it makes the electrical and also the pneumatic connections and disconnections automatic. However there is no standard for the placement of these electro-pneumatic connections. Some rail companies have them placed on the sides while others have them placed above the mechanical portion of the Schaku coupler. The main disadvantage to the Scharfenberg coupler is its low maximum tonnage, which makes it unsuitable for freight operations.
Small air cylinders, acting on the rotating heads of the coupler, ensure the Schaku coupler engagement, making it unnecessary to use shock to get a good coupling. Joining portions of a passenger train can be done at very low speed (less than 2 mph or 3.2 km/h in the final approach), so that the passengers are not jostled about. Rail equipment manufacturers such as Bombardier offer the Schaku coupler as an option on their mass transit systems and their passenger cars and locomotives. In North America all the trains of the Montreal Metro are equipped with it, as are new light rail systems in Denver, Baltimore and New Jersey. It is also used on light rail vehicles in Portland, Minneapolis, the Vancouver Skytrain, and the Scarborough RT in Toronto. It also equips all the dedicated rolling stock used for the shuttle services in the Channel Tunnel.
Due to the rush to dieselise and electrify, the United Kingdom ended up with a variety of incompatible couplings and electrical connections. The latter were categorised as yellow triangle, blue square, and so on. This has nothing to do with the physical connection of vehicles. Coupling codes, as they were known, became relevant only if multiple working of locomotives or multiple units was required.
The Swedish-made Dellner coupling is a proprietary version of the Scharfenberg coupler, connecting both vehicle, pneumatics and electronics at the same time. The patented energy absorption D-BOX technology allows coupling at speeds of up to 15 kilometres per hour (9.3 mph) with no structural damage, and up to 36 kilometres per hour (22 mph) with deformation but with the vehicles remaining on track. The patented D-REX system provides Ethernet high speed data connection at speeds of 100 Mbit/s.
Sometimes a wagon with one coupling system needs to be coupled to wagons with another coupling type This may be needed when taking metro rolling stock from its manufacturer to the city where it is to be used. There are two solutions:
Only some kinds of couplings coexist on the end of a wagon at the same time, because amongst other reasons they need to be at the same height. For example, in the Australian state of Victoria, engines had the AAR coupler, with buffers, and the chain mounted on a lug cast into the AAR coupler.
A match wagon or match truck (also known as a "barrier vehicle" / wagon in Britain and "transition car" in North America) has different kinds of couplings at each end. If a pair of match wagons is used, a rake of wagons using coupling A can be inserted into a train otherwise using coupling B.
A coupling adaptor or compromise coupler might couple to an AAR coupling on a wagon, and present, for example, a meatchopper coupler or rapid transit coupler to the next wagon. Such an adaptor might weigh 100 kg (220 lb).
Automatic couplers like the Janney are safer in a collision because they help prevent the carriages telescoping. British Rail therefore decided to adopt a Janney variant for its passenger carriages, with the coupler able to swing out of the way for coupling to engines with the traditional buffer and chain system.
In New South Wales, sets of carriages were permanently coupled with a fixed bar, since the carriages were disconnected only at the workshops. Freight cars are sometimes coupled in pairs or triplets, using bar couplings in between.
Couplings are needed for any continuous braking systems.
Electronically controlled pneumatic brakes (ECP) need a method of connecting electrically adjacent wagons, both for power and for command signals, and this can be done by plugs and sockets, or by very short range radio signals.
A draw gear is the assembly behind the coupling at each end of the wagon to take care of the compression and tension forces between the wagons of trains. Early draw gears were made of wood, which was gradually replaced by steel.
Some couplers may not have a draw gear.
On model railroads couplers vary according to scale, and have evolved over many years. Early model trains were coupled using various hook-and-loop arrangements, which were frequently asymmetrical, requiring all cars to be pointing in the same direction. In the larger scales, working scale or near-scale models of Janney couplers were quite common, but proved impractical in HO and smaller scales.
For many years, the "X2F" or "Horn-Hook" coupler was quite common in HO scale, as it could be produced as a single piece of moulded plastic. Similarly, for many years, a "lift-hook" coupler known as the Rapido and developed by Arnold, a German manufacturer of N-scale model trains, was commonly used in that scale.
The chief competitor of both these couplers, more popular among serious modellers, was the Magne-Matic, a magnetically released knuckle coupler developed by Keith and Dale Edwards, and manufactured by Kadee, a company they started. While they closely resemble miniature Janney couplers, they are somewhat different mechanically, with the knuckle pivoting from the center of the coupler head, rather than from the side. A steel pin, designed to resemble an air brake hose, allows the couplers to be released magnetically; the design of the coupler head prevents this from happening unless the train is stopped or reversed with a mated pair of couplers directly over an uncoupling magnet. An earlier, mechanically tripped version of the design had a straight pin extending down from the knuckle itself, which engaged a diamond-shaped mechanical "ramp" between the rails, which had to be raised above rail height when uncoupling was desired.
Once the Kadee patents ran out, a number of other manufacturers began to manufacture similar (and compatible) magnetic knuckle couplers.
Recently, an exact-scale HO model of the AAR coupler has been designed and manufactured by Frank Sergent. This design uses a tiny stainless steel ball to lock the knuckle closed. Uncoupling is achieved by holding a magnetic wand over the coupler pair to draw the balls out of the locking pockets.
In O scale, an exact-scale working miniature version of the "Alliance" coupler was manufactured from the 1980s by GAGO models in Australia. Since 2002 it has been marketed by the Waratah Model Railway Company. European modellers tend to use scale hook and chain couplings.
In British 00 scale (similar to H0 scale) models the 'tension lock' coupler developed by Tri-ang is standard. This is similar in operation to the meatchopper type of coupling. Remote uncoupling is possible by using a sprung ramp between the rails. The design of the hooks is such that the couplings will not uncouple when under tension (instead depressing the ramp). When the train is pushed over the ramp, it will lift the coupling hooks as the train passes over. By halting the train over the ramp, it is split at this point. While it works well, it is often seen as ugly and obtrusive (although smaller designs are available, these are not always fully compatible with other models) and many British modellers prefer to retrofit either Kadee types or working hook and chain couplings.
A recent development is an interchangeable coupling which plugs into a standardised socket, known as NEM 362 and which can be easily unplugged as required. This allows the modeller to easily standardise on whatever coupling is desired, without individual manufacturers needing to change their coupling type.
A comparison of coupler types was published in "An introduction to Couplers".
Different kinds of coupling have different accident rates.
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