When you begin by gently fisting your partner, imitating the connecting road of a steam train(link from the engine to the wheels). Slowly picking up speed much like a train itself. Once you have reached maximum speed you can manage, you yell CHOO CHOO, spouting two glorious arks of semen verticallyin the air to mimic the puff of steam.
When two homosexuals choose to play the rodent game (sticking a rodent into one's bum) and they put a tube between the two, the partner with the rodent inside him farts in order to try and get the rodent to run into his partner's bum.
Wow! did you see that hamster run from Frank's bum to Steves? that was quite a choo choo train