I recently acquired a Prairie Flyer set with a 2-4-2 "Old Timer" Locomotive. Set the track up under the tree, put the Loco on the track and it gets power, but doesn't move under its own power like it should. It "bangs" and skips.  

Took the lower chassis apart and on the rear drive wheels, there is a gear on a serrated axle shaft. The gear has a split in it between two teeth, and it doesn't grip on the axle like I feel it should. The Gear can also slide completely off of the serrated section.  I'm assuming this isn't how it should be and is the route of my problems? If so, does anyone know part numbers or parts I can use to fix this? Thanks

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