attaching o gauge can motor to gear box on MTH diesel

make sure the motor's electrical tabs are not touching the shell on the motor! I would add heat shrink to be sure.

Turn the flywheel several rotations by hand and feel for the teeth to mesh smoothly. The mounts must be tight!

Are you swapping components? do you have the right gears matched up?

Ill try turning the flywheel. Thanks. Are these electrical tabs on the outside of the can motor? I did find one wire which was getting frayed by a mounting screw and replaced that.

Yes, the tabs are what the two power wires attach to.

Are you changing out the motors? or servicing the stock set-up??

If you operate a lot, things wear out.

We found that, with good lubrication, worm wheels lasted 3 years under museum operating conditions.  You can tell with a good visual inspection.

Motor bearings - critical for gear mesh - I think made it to six years, and axles/axle bearings to ten.

I was just looking for an electrical short. I will try turning the fly wheel to get a proper mesh.