Similar thing happened to my lionel 18301 trainmaster diesel loco. That era they used plastic gears on the drive axle. The plastic gears didn't age well. The "unrun" for 20 years made it a couple of loops before the plastic gears split. It now has brass gears on both sets of drive axles, similar to the trainmasters from the 1950's.
Split brass gears is unusual. Tolerances at the factory may have been off.
