Buffers and chain coupler

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Buffers and chain couplers (also known as "buffers and screw", "screw", and "screwlink") are the de facto International Union of Railways (UIC) standard railway coupling used in the EU and UK, and on some railways in other parts of the world, such as in South America and India, on older rolling stock. Buffers and chain couplers are an assembly of several devices: buffers,^[1] hooks and links, or turnbuckle screws.^[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. That limits slack, and lessens shunting shocks in moving trains. By contrast, vehicles fitted with the semi-automatic Janney Type E coupler can experience significant jarring during mating and shunting. Very early rolling stock had "dummy buffers", which were simple rigid extensions of the frame, but they were improved with the use of enclosed mechanical, then hydraulic, springs to damp possible jarring. Each chain incorporates both a hook and a turnbuckle.