Buffers and chain coupler

[2] On the modern version of the couplers, rail vehicles are mated by manually connecting the end link of one chain which incorporates a turnbuckle screw into the towing hook of the other wagon, drawing together and slightly compressing the buffer pairs, one left and one right on each headstock.

By contrast, vehicles fitted with the semi-automatic Janney Type E coupler can experience significant jarring during mating and shunting.

The slack between vehicles coupled in this way was very convenient when starting heavy trains with a relatively low-powered locomotive on the level or on a rising gradient.

The disadvantage of that convenience was that the guard could be badly thrown about as the train changed speed due to the inter-wagon gaps opening or closing.

[4] An improvement on the loose-coupled train is the "Instanter" coupling, in which the middle link of a three-link chain is specially triangular shaped, so that when lying "prone" it provides enough slack to make coupling possible, but when the middle link is rotated 90 degrees, the length of the chain is effectively shortened, reducing the amount of slack without the need to wind a screw.

Three-link and Instanter couplings can be operated entirely from the side of the wagons, using a shunter's pole, which has a hook on the end, and is safer when shunting is being done.

Ordinary three-link couplings have been superseded by instanter, screw or buck-eye couplers in UK freight trains today.

On some narrow-gauge lines in Europe, and on the Paris Metro, a simplified version of the loose-coupler is used, consisting of a single central buffer with a chain underneath.

Although careful track design makes this a rare occurrence, a series of derailments at Stuttgart Central Station in 2012 were caused by buffer-locked wagons.