I'm just getting into trains again and I've been tinkering around with some old Jupiter 4-4-0s just to see how they work and come apart. The worm gear came out and doesn't seem to want to stay in place now. I thought maybe it was broke but I can't see a broken shaft or anything. What am I missing? Before I started to "work on it" the wheels would turn as long as I was turning the drive shaft. They would not turn freely by themselves. Can you replace the worm gears in these older models?