I finally got time today (I bought it 2 weeks ago) to put some track together for her maiden voyage. After about eight runs around the track it started derailing. I noticed the middle 2 axles no longer have any side movement like they did in the begining. If I let it be for a while they would loosen up a little and it would stay on the track for a round or two. It's like they get hot and want to freeze up.

Anyone know of this problem and if there's an easy fix?

Thanks