I used to believe that trains were full of small horses inside. And those small horses' legs were responsible for the train movement.
A childhood belief about trains

others believed things like this
- When I was little I thought cows married horses, cows were all female and horses were all male. — Sheila
- I used to believe that horseradish
was made from horses.— Dee - I used to believe cows were ugly horses— Marlene
- When I was ten, I used to believe that the horses didn't sleep.— Jared
- I used to believe, as a child, that horses were male cows.— Martin Taylor
- i use to beleive that horseraddish came from horses shoesAnon