A euphemism for a gangbang or a gang-rape.Some here are afraid to admit that it refers to rape and say it is VOLUNTARY.It is safe to say it refers to either group sex or gang rape.This word is usually used by gangstas and thugs.
Watch the TRAINING DAY scene when alonzo stops the white kids in the gang neigbourhood.
Denzel W(Alonzo):I'ma make you walk home. I'ma let the homeboys run a train on your girlfriend. You know what a train is, don't you?
some actor(driver):(wimpy voice)Yes.