I didn't even notice based on my picture that peice had 2 separate gears and you'll see why when I upload the pic
Seller left this in pretty neglected shape I know, but yeah it hid the separation. Now that I see it, I don't think your cracked gear is the same shape as one you would find in a flat drive unit Trackmaster 1 train; I'm almost certain now. I haven't researched gears like that to buy replacements for myself so I don't have a link to those replacement gears. Looks like you have 2 solutions.
Either to superglue that gear together (compress the gear teeth as tightly as you can before the glue dries, also make sure you don't use to much glue, you don't want dry residue around the teeth otherwise you'll hear noises when running),
Or you can buy the cheapest Trackmaster/TOMY road vehicle and swap parts (I'd go for cheapest road vehicle). You ARE rolling the dice when buying used but that gear being cracked is not a common occurrence for what CAN go wrong with used road vehicles.
But again there may be other solutions out there, this is just what I would do with my experience.
