To me, there are actually 2 correct answers.
The full phrasing is...."chugga chugga.....chugga chugga.....chugga chugga......CHOO CHOO".......that's the full version for when you need to get the train moving.......getting progressively faster.
The shortened version, and the correct version, is "chugga chugga.....choo choo".
Your thoughts??!?!
Here's some of the controversy from the interwebs........