I have a Bachmann 4-8-4 steam loco, the drive wheels are slipping out of the gears. I am not familiar with locos, can you tell me how to fix this problem?
There really is not a fix, just replacement.
Cyanopoxy http://www.mrhobby.com/Cyantips.html (http://www.mrhobby.com/Cyantips.html) might work, if you can line everything up perfectly - the downside is that a whole new locomotive of improved construction would not cost a whole lot more.
You did not state which 4-8-4 you had, which means my answer is not precise. There may be different answers for different models. If the model is in warranty, telephone the service department and talk to them about your options.
If you have the old Bachmann 484 it is junk and not worth much except in a display case
The newer DCC model should be OK Mine runs real well