You’ve got to drop the bottom plate that holds the bearing blocks in place in order to drop out the motor and gear assembly. A bunch of little screws to either side of each axle into the frame. Careful to make sure when you drop this that the springs that are between the top of the bearing block and the top of the frame don’t get lost. With that plate out of the way and the siderods removed (hint: do this first), you should be able to drop the motor and gearbox assembly. Next step is to remove the drivers. There’s a small cap over the center of each driver that hides a screw. Gently pry that off with a flat-head screwdriver to reveal the phillips head screw beneath. Remove that screw, and gently pull the wheels off the axle. The drivers are quartered, so you don’t need to worry about that. Remove the bearing block, paying attention to its orientation. (And don’t lose the springs.) Then you’ll want to remove all the eccentrics and valve gear stuff. Hint - clamp or tape all that stuff together so it doesn’t come apart. That way you can easily put it back together once you’re done.
Now, you should have the motor, gearbox, and axle. I forget exactly how to take the gearbox apart, but I think it’s six or so screws from the side that expose the gears. With that open, you can inspect the gears to figure out where the problem is. 99% of the time, it’s going to be the main gear on the axle that’s split. You have a few options.
-
If it’s just the hub that’s cracked and loose on the axle, but the outside part of the gear is intact, you can get some 3/8" brass tubing, cut some thin slices off, and reinforce the hub of the gear with that. A little JB Weld and you’re good to go. I did one that way and it’s held up nicely since.
-
Call Bachmann service and get a replacement gear (if they have them). This should be around a $3 gear. Once you get that gear, get some 3/8" brass tubing, cut some thin slices to reinforce the hub, JB Weld in place, and reassemble. (You don’t “need” to do that, but you’ll just have to open the gearbox up down the road to do so or replace the gear again.
-
Order a replacement Delrin gear from Northwest ShortLine (Part # 2223-6, $12.95). This is a Delrin gear which has proven to be quite a bit stronger than the stock gear. To my knowledge, no one makes a metal replacement gear.
If it’s not the main axle gear that’s broken, then your only option is to call Bachmann and get a replacement for that gear, or a replacement motor/gear assembly.
Of course, if you’re adventurous and don’t mind doing some cutting and fitting, I’ve pondered fitting a BBT B’mann 2-8-0 replacement motor/gearbox in one of my 2-6-0s. I’m pretty sure there’s ample room in the boiler, but you’d have to cut away some of the bottom so the gear tower could extend up. You’d also probably lose the operating reverse gear (no big loss, if it means the loco runs reliably.) Some day, I’ll splurge for one and experiment. (Or, Barry will develop a replacement gearbox, hint, hint. 
Good luck!
Later,
K