1. A train that is in working order, or is at the present time running.
2. Slang for working hard without any rest periods.
3. Slang for a group of people, usually men, possibly part of a club or group that have all had sexual intercourse with one woman/man, either during an orgy, or sequentially coupling with her/him over a short period of time. No compensation, except temporary popularity of the person who has been run train on, is ever given, and they are usually not held in high regard by their sexual partners post coitus.
2: "I've been running train on this project to get all the loose ends tied back up."
3: Steven : "I'm going out with Nicki (or random name here), she seems really outgoing."
Todd: Laughs, "Yeah she is 'outgoing'. She was a bat girl when we were in high school so she could go on away trips with the baseball team. They were running train on that chicken head for two years before she caught on what they really thought of her. If you hit that I'd wear a rubber, or maybe five."