It isn't uncommon for the plastic gear to fracture as discussed here, but pictures/better description could help diagnose your specific issue:
@bmoran4 posted:I believe the gear is 610-8806-576, or that this gear should atleast suffice. They are not available from Lionel proper, but @Harry Henning of Henning's Trains, a forum sponsor, has it in brass: https://hennings-trains.shopli...-brass-13647485.html
You will want a wheel puller and you will need to get crafty with a cobbled backing plate (I've been known to use a small socket) to position the gear on the axle. You can use a vice to gently press the wheels back onto the axle (although a proper press may be preferred).
