"Poo-Choo Train" is a song about the enchanted locomotive made out of feces. It was used to ride to the North Pole in the Season Six episode, "Red Sleigh Down".
Background
Eric Cartman and Mr. Hankey sing about the train as they ride with Stan Marsh and Kyle Broflovski.
Lyrics
Mr Hankey:
Poo choo train's layin' down its tracks with a-
Poo Choo Train:
POO CHOO!
Mr Hankey:
All the way and back.
Cartman:
Poo poo train is my favorite thing, spreading Christmas joy as we ride and sing!
Mr Hankey: [slower tempo]
Christmas time wouldn't be the same without hugs and kisses and a poo choo train!