railway coupling
英[ˈreilwei ˈkʌpliŋ]美[ˈrelˌwe ˈkʌplɪŋ]
A coupling (or a coupler) is a mechanism for connecting rolling stock 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.