WABCO H2C

A coupling (or a coupler) is a mechanism for connecting 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 or draw 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 basic type of coupling on railways following the 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 over-ran a slowing locomotive.

The simple chain could not be tensioned and this slack coupling allowed a lot of back-and-forth movement and banging between vehicles. Acceptable for mineral wagons, this gave an uncomfortable ride for passenger coaches, and so the chain was improved by replacing the centre link with a turnbuckle that draws the vehicles together, giving the 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.

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.

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Wikipedia