Will, when the drivers are in motion to permit the loco to move forward, and if the crank pin were kept from rotating with the driver, then the relative effect is that the gripped crank pin is being forced to back itself out since it is spinning counter clockwise relative to the rotation of the drivers. This, to me, can only mean that something else contacting the pin is acting like a vise and preventing it from rotating forward. The only thing it could be is something surrounding the outer head, and that would have to be one of the rods. It will be difficult to discern which it is, but if you invert the locomotive completely, and have a good look at the parallel arrangement of all the rods, you may find what the gentleman above suggests is the case...a rod, or rods, sufficiently bent that they actually make contact. Another possibility is that there is an accumulation of grime, semi-dried or waxy oil, and very probably some wound pet hairs, human hairs, fibres from clothing and bedding, you name it....laundry lint, perhaps. All that stuff could eventually form a credible bond, and not a lubricating medium, for any one or all pivoting rod heads.
Really good light, a magnifyer (about 6-10 times) and tooth picks, the ones with two pointy ends, would be your first method of attack. Once you have them cleaned, do the others, and those on the other side. Lube them with ATF (Dexron II or III) lightly, and you will have your locomotive back...I hope and expect. Might be a good time to do the whole shebang...motor bushings or bearings if they can be cleaned, and do the transmission.
-Crandell