I have a large scale Anniversary Edition set with a 4 - 6- 0 locomotive.  While I have not used the set in several years, it has always run well, and has not been overused.

I set up the train set this week, and the drive train on the locomotive seems to be slipping with all of the cars attached.  If I start the locomotive slowly, and bring it up to speed carefully it will get around the track.  But, if I push the speed up too fast, the motor runs, but does not drive the locomotive unless it's given a push to get started.

I opened the drive train section, and the gears seem to be tight enough, with no evidence of a stripped gear.

Does anyone have a thought on what the problem may be?

Thanks in advance - I want to show it off to my 18 month old grandson as soon as possible.