I have a Santa Fe locomotive that connects to the track and receives power but can barely move - very sluggishly. I took the shell off to inspect it and while doing so I noticed a small black rod that was jammed in between the wheel and the wheel cover. Thinking this was my problem, I pulled it out and re-ran the train but there was no improvement. I think the rod may actually be part of the transmission assembly and somehow fell out through the axle connection area. It's hard to inspect the guts of the loco because of the top circuit board.
That is probably a drive shaft going to a truck. Sounds like one truck is dragging the other truck. Both trucks receive power from the motor from a drive shaft. Look at the paper work that came with your loco.
If you do not have the paper work, Look in the information page for your loco.
You will have to lift the PC board.
Rich