My gordon had the EXACT same issue, will take him apart and post pics of what the issue was for me. It was the switch would not engage the gear before Gordon's drive wheels, and he made a horrid racket (he always did, it just got louder when the issue began) and it started at a train show! Thankfully, Edward made it look like the TV show where edward pushed gordon up the hill. And three years later, after I got back into this awesome hobby, I fixed him. It was a surprisingly easy fix but it sacrifices free rolling when the loco isn't on.
Proud owner of Tomy/Plarail trains. 