Trains that 'chug'

I've had a chugging issue too with 1 or 2 of my Trackmasters in the past (used) and just like Super, It was so long ago and I've also done a plethora of things to the engine as one does when buying used engines in general that I don't remember what the cause of it was I'd be guessing because of my limited experience with chugging engines. But maybe check if the running wheel axels gear is cracked or otherwise loose (because of said crack). That is, if you don't choose to return it.