I knew that they worked, they are actually the same size, however the C size offers better performance. I have a few locos that I use a AA instead of a C however I think it's important to make a spacer or else it may ge annoying to have to reposition the battery. I think the problem could be the wires between loco and tender, as if they are not long enough they could restrict the motion of the drawbar
The magic of the GWR