noun
automatic coupler
A device that automatically connects railway cars when they are pushed together, without manual intervention.
日本の鉄道車両の多くは自動連結機を装備している。
Many Japanese railway cars are equipped with automatic couplers.
連結器 is the general term for a coupler, while 自動連結機 specifically refers to an automatic coupler.
Compound of 自動 (automatic) + 連結 (coupling) + 機 (machine). The term is a straightforward technical coinage.