I have a Bachmann 2-8-4 Berkshire locomotive that has been working fine, untill recently, when the gears started to slip. When I put 5 or more cars behind it it makes a "cccrrrccccrrrccc" sound, and the wheels dont turn. I dissasembled the loco, but could not find the problem. All screws were in tight, and everything was greased/oiled.

Please help me fix this problem!!