New, Duracell batteries, tested and all good, in remote and train. All lights and sounds work. Control board is clean, welds all seem connected well but I can't see the welds to the motor. Motor functions very slowly at first, but stops and one gear seems locked in place, but I see nothing for it to be locked on. Gears are very well greased and all other gears function as normal. Is it the motor? Or the gear? How do I fix this thing?