The drive train on my N Scale GP40 works fine when taken out of the loco and powered directly with jumpers from my power pack (DC) The wheels are driven by the motor as one expects. However, and the reason for disassembly, when in the loco frame and on the track, the wheels don't turn and the motor whines away ineffectually.
Update: Yesterday Dec 8th I did get some actual running on the track but it was very noisy and jerky. So the wheels do indeed turn. However this is not at all smooth.
Have any of you had problems like this with the Bachmann GP40 I don't know how old the model is. It says Bachmann on one side of the fuel tank, and Made in China on the other. It has a split frame coloured black. See photo. https://imgur.com/a/LVZ9woF
Unlike on Atlas N Scale truck, there seems to be no easy way to take the trucks apart to further diagnose the issue.
Any helpful suggestions welcome, thanks.