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.