MTH loco repair

Check for crap in the gears, it's one of the more common reasons.

If the side frames are attached from the bottom with two screws, remove them and inspect the traction tires, gears, the screw in the bottom of the truck that attaches the motor (be sure it’s tight).  If it runs with the sideframes off, check for interference between the brake shoe and wheel with the sideframe reattached.

Thanks for the tips, guys. I removed the shell and inspected the gears and found out that a couple pieces of Woodland Scenics "coarse ballast" had indeed got stuck in the gears. After digging them out, it runs fine now!