When a group of guys (usually 7 or more) run a train on an Asian girl. A train usually involves 7 or more guys having sexual intercourse with 1 girl. In New York City the 7 train runs from Time-square in Manhattan to Flushing in Queens. Ethnic Chinese constitute an increasingly dominant proportion of the Asian population and as well as of the overall population in Flushing; hence the definition of 7 train.
Example: "Dude our Mandarin teacher Ms. Wang is a whore. I heard the football team ran a 7 train on her in the locker room."
Example: "Yo, me and my boys caught the yellow fever last night. We ran a 7 train on the Asian girl from our Korean History class."
Example: "Why are you surprised she is a slut? She is from Flushing and she loves to ride the 7 train. What did you expect stupid?"