A group of guys stand in a line outside a room. One by one, they enter the room to have sex with one girl inside. The line must have at least 7 guys to be qualified as a train.

"What's this line for?" "We're running a train! Go to the back of the line."

To "run a train" in sexual terms means that a group of men wait to have sex with one girl. Usually it does not qualify as a train unless there are more than 7 men involved but the term train is used to describe any number that is more than 1 with a female. The female has sex with a man and after he cums another man enters the room to have sex with the female and so forth. It is only considered a train when the woman is fully aware of the situation and allows for the men to have sex with her. Otherwise it would be considered rape. Differs and is often confused with gang bang. Train involves sex with one person at a time while gang bang is when a group of males having sex with a female at the same time.

"Shorty right there is a true freak. Me and my boys ran a train on that trick the other day."

Something Bruno Mars would jump in front of.

I'd jump in front of a train for ya

Where a usually slutty girl, often drunk, gives it up to a somewhat large number of guys, one after another after another. Metaphorically speaking, the girl is like a urinal, just instead of urine, the guys using her are relieving themselves a little differently.