Since if one person "rails" another, then two persons committing the act on one person would therefore connect the hypothetical rails, resulting into a hypothetical train running through the receiver.
"Yo bro, let's run a train on that bitch from 48th Ave."
"Bruh we should run a train on the bitch from 10th Downing St."
The freight train is when your having sex in a car, you put the girls head in the window but not to the point where it chokes her, ass in seat, then you put a cigarette with duct tape around her mouth, then every time the male thrusts, smoke comes out of her nose like a freight train
Harrison "dude did you have sex with that girl last night?"