I bought a Bachmann GP50 that I'm trying to turn into a dummy engine to pull behind like diesels. I already had an older model that I pulled the motor and drive rods/worm gear out of and it pulls fine behind a GP38. This new GP50 won't be pulled except for the strongest of motors (SD70 and such). Could too much grease in the wheel assemblies be dragging it down too much? I've cleaned it as well as I could, removed the gearing and motor, and it's actually lighter than the older GP50 that glides well. Any ideas?