Train is a a term for a group sex position involving multiple people whose sexual acts connect their bodies in a train-like fashion. The participants involved in a train may be engaged in oral sex, analingus, anal sex, or penis-in-vagina intercourse. The participants in a train may be of any gender or sexuality.
A train can also be called a sex train. Running a train is also a common slang used to indicate that a person has sex with multiple people, one after the other.