I was carefully observing the roll of a steam locomotive today. Ther was one point near the end of my "Bryson" yard and sometimes the train would stall and the wheels would spin. Now I know a bit about standing in place and spinning wheels in my real life so I started looking for the problem.
I tracked down where the safety rails on a bridge over the river came close to the center of the track. Seems one of the couplers on one car was catching at this point. First, I thought the coupler was off (low) by a thousandth of an inch or so. Unfortunately, it wasn't this easy. The safety rail was high. So back to trackwork.
Next I noted where the train cars seemed to jump. Again, one rail was high by about a thousandth of an inch. Ripping up track that I thought was set perfectly. I always seem to be making adjustments in my life. Small adjustments, large adjustments. Size does matter.