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, 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
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