I wish I had tried to remember the words, learned from someone from South Wales I think, before I read this. I think it was

Early in the morning
On the way to London
See the little engines
All in a row
See the engine driver
Turn his little handle
Puff puff toot toot
Off we go.

However, I do remember that I learned it a four part round which was made all the more entertaining by it having 'actions' for most lines. These are fairly obvious and better with 'puff puff toot toot' than with some of the other versions above.