A coupling or coupler is a mechanism, typically located at each end of a rail vehicle, that connects them to form a train. The equipment that connects the couplers to the vehicles is the draft gear or draw gear, which must absorb the stresses from coupling and the train's acceleration.
Throughout the history of rail vehicles, a variety of coupler designs and types have been developed worldwide. Key design considerations include strength, reliability, ease and efficiency of handling, and operator safety. Automatic couplers engage when the cars are pushed together. Modern versions not only provide a mechanical connection, but can also couple brake lines and data lines.
Different countries use different types of couplers. While North American railroads and China use Janney couplers, railways in the former Soviet Union use SA3 couplers, and the European countries use Scharfenberg and screw couplers. Challenges and complications arise when coupling vehicles with different couplers. Barrier cars, also called match cars, cars with dual couplers, or adapters are used to accomplish this task.
Nomenclature Compatible and similar couplings or couplers are frequently referred to by widely differing makes, brands, regional names, or nicknames, which can make it confusing to describe standard or typical designs. Dimensions and ratings noted in these articles are usually of nominal or typical components and systems, though standards and practices also vary widely with railway, region, and era.
Buff: when the consist (one or more cars coupled together) of cars is in compression; opposite of tension.
Buffers and chain
The basic type of coupling on railways, following British tradition, is the buffer-and-chain coupling. A large chain of three links connects hooks on the adjoining wagons. These couplings followed earlier tramway practice but were made more regular. Buffers on the frame of the wagon absorbed impact loads, as the train overran a slowing locomotive.
The simple chain could not be tensioned, and this loose coupling allowed a great deal of back-and-forth movement and bumping between cars, as well as jarring when trains started. While acceptable for mineral cars, this coupling made for an uncomfortable ride in passenger coaches, so the chain was improved by replacing the center link with a screw threaded left-hand on one side and right-hand on the other. In the center of the screw is the handle housing with a hinged ball handle attached. This turnbuckle style arrangement allows the vehicles to be pulled together by tightening the screw with the attached handle. Typically, the screw is tightened until two threads remain next to the handle housing. A support is attached to the trunnion nut on the coupling link side to rest the handle of the screw to prevent loosening of the screw while the coupling is in use. The official name of this type of coupling is screw coupling or UIC coupling according to the European standard EN 15566 Draw gear and screw coupling.
A simplified version of this, called an Instanter coupling, quicker to attach and detach, still used three links, but with the centre link given a T-shaped slot. This could be turned lengthwise to lengthen it, allowing coupling, then turned vertically to the shorter slot position, holding the wagons more tightly together.
Higher speeds associated with fully fitted freight made the screw-tensioned form a necessity.
The earliest 'dumb buffers' were fixed extensions of the wooden wagon frames, but later spring buffers were introduced. The first of these were stiff cushions of leather-covered horsehair, later steel springs, and then hydraulic damping.
This coupling is still widespread in Western and Central Europe and in parts of Northern Africa, the Middle East, and South Asia.
Link and pin
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 forest railways. While simple in principle, the system suffered from a lack of standardization in the size and height of the links and 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 tube's end 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: It made a loose connection between the cars, with too much slack action. There was no standard design, and train crews often spent hours trying to match pins and links while coupling cars. Crew members had to go between moving cars during coupling, and were frequently injured and sometimes killed. The links and pins were often pilfered due to their value as scrap metal, resulting in substantial replacement costs. When a car happens to be turned 180 degrees, one would have to look for a link. Railroads progressively began to operate trains that were heavier than the link-and-pin system could cope with.
In Britain, link-and-pin couplers were common on narrow-gauge industrial and military railways. Eventually, they evolved into a form that could be reliably coupled when the train was stationary.
The Panama Canal mules, the locomotives used to guide ships through the locks of the Panama Canal, have link-and-pin couplers and side buffers. This design was chosen so that these normally solo-operating locomotives could be coupled to another locomotive in the event of a breakdown. On a straight track, the link-and-pin coupler is used. Since the vertical curve between the straight track sections and the ramp between the lock chambers has a very small radius, the height difference would be too great for a link and pin coupler, so the locomotives must be pushed through these sections uncoupled by using the side buffers. They have an extra-high buffer plate to prevent the buffers from locking in tight vertical curves.
Balance lever coupling
The balance lever coupling, also known as a central buffer coupling with two screw couplings, is a coupler commonly used on narrow gauge railroads with tight curves. By swapping the pulling and pushing devices, the standard screw coupling used on standard-gauge railroads became a center buffer coupling, with one screw coupling on each side of the buffer. The screw couplers are connected to a compensating lever that pivots on a vertical trunnion on the center buffer rod, allowing an even distribution of tractive forces between the two screw couplers.Bruno Lämmli: Zug- / Stoßvorrichtung. Auf: Lokifahrer, 2021.
Albert coupler
To avoid safety issues, Karl Albert, then the director of the Krefeld Tramway, developed the Albert coupler in 1921. The Albert coupler was designed as a key-and-slot coupler with two pins. Vehicles to be coupled were pushed together, both couplings moving to the same side. One pin was inserted, then the vehicles were pulled to straighten the coupling, and the other pin was inserted. This operation required less exact shunting. Due to 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 with automatic couplers. But even in modern vehicles, Albert couplers get installed as emergency couplers for towing a faulty vehicle.
Miller hook and platform 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.Miller Hook The Miller platform (and hook coupler) was used for several decades before being replaced by the Janney coupler.
Norwegian
The Norwegian coupler consists of a central buffer with a movable hook that drops into a slot in the central buffer. There may also be a U-shaped safety catch on the opposite buffer that is flipped over the top of the hook to secure it. The safety device may also be a chain with a ball-shaped weight at the end that is thrown over the hook to hold it in place. On railways where the rolling stock always faces the same direction, the mechanical hook can be on one end of the wagon only. Not all Norwegian couplers are compatible with one another as they vary in height and width, and may or may not be limited to one hook at a time. The traction force limit is typically 350 kN. Sometimes the Norwegian coupler is supplemented with auxiliary chains.
The Norwegian coupler is also known as the Lloyd coupler, named after its British manufacturer, F.H. Lloyd & Co. Ltd, near Wednesbury, or as the meat chopper coupler, named after the shape of the movable hook. The Norwegian coupler allows sharper curves than the buffer-and-chain coupler, which is an advantage on narrow-gauge railways, where low speeds and reduced train loads allow a simpler system. The Norwegian coupler is found only on narrow gauge railways of , or less in Great Britain and its former colonies. For example, it is used on the Isle of Man Railway, the Western Australian Government Railways, in Tanzania, on the Ffestiniog Railway, on the Lynton and Barnstaple Railway, and on the Welsh Highland Railway,
Radial couplers Two versions of the radial coupler were used in South Africa. One, the Johnston coupler, commonly known as a bell link-and-pin coupler, was introduced in 1873 and is similar in operation to, and compatible with, link-and-pin couplers, but is bell-shaped with a circular coupler face. The other, the bell-and-hook coupler, was introduced in 1902 and is similar to the Norwegian coupler, but also has a circular coupler face and a coupler pocket that is open at the top of the coupler face to accommodate the drawhook.Suid-Afrikaanse Vervoerdienste (South African Transport Services) (1983). Passassierswa- en Trokhandboek (Passenger Carriage and Truck Manual), Vol 1, Hoofstukke 1-15 (Chapters 1-15). South African Transport Services, 1983. Chapter 13.
Johnston coupler
The Johnston coupler, commonly known as a bell link-and-pin coupler from its bell shape, was first introduced in the Cape of Good Hope in 1873, following the establishment of the Cape Government Railways (CGR) in 1872 and the decision by the Cape government to expand the railways into the interior and to convert the existing tracks from to Cape gauge. All new Cape gauge locomotives and rolling stock acquired from 1873 were equipped with these or similar couplers, beginning with the CGR 0-4-0ST of 1873, a construction locomotive named Little Bess.
The Natal Government Railways (NGR), established in the Colony of Natal in 1875, followed suit, and all locomotives and rolling stock acquired by that railway were equipped with Johnston couplers, beginning with the NGR Class K 2-6-0T in 1877.Espitalier, T.J.; Day, W.A.J. (1944). The Locomotive in South Africa - A Brief History of Railway Development. Chapter III - Natal Government Railways. South African Railways and Harbours Magazine, May 1944. pp. 337-340.