A term used to represent the speed and efficiency at which gossip travels. It draws to mind a train carrying rumours on it and depositing these rumours to everyone it passes.
If you find yourself in a situation where gossip is traveling fast, for example, a camp or a small high school, or you find out a news/gossip item that is fairly recent, you then say "Rumour Train!" noting that the piece of gossip is traveling at the speed of a train.
You may optionally say "Rumour Train, WOO WOO" afterwards to emulate the sound a train containing rumours would sound like.
Senior Camper 1: Did you hear Noah and Rachel are foolzing again?
Senior Camper 2: When did it happen?
Senior Camper 1: Last night!
Senior Camper 2: Rumour TRAIN! WOO WOO!
Staff member: I heard you and Miranda were making out at Chapel point after evening program.
Camper: HOW DID YOU HEAR THAT?
Staff member: Rumour Train!