A device used for joining two rail cars so they snuggle up and follow the leader. The locomotive that is.
The first couplers used in North America were what is called link in pin. A simple link, like used to make chain, put into a slot and held there with a big steel pin. A railroad carman had to hold the link while the locomotive backed the train. Needless to say, there were many amputated fingers.
To overcome this deficiency in the mechanics of link and pin couplers many inventions were made. Most were not effective in operation or cost. A veteran of the civil war named Eli H. Janney invented a knuckle coupler which was both. US patent #138405 The new Master Car Builders Association convinced the manufacturer of Mr Janney’s coupler to release some of the patents that would allow others to make compatible couplers. So we have the current coupler, evolved of course.
There are other systems such as the British buffer and links, the ABC coupler, and the chopper or Norwegian coupler. The cjopper coupler is still used on many railroads even today. The other major coupler is the Willison/SA3 coupler used by Russia and many others.