A Contact Problem

I have made some layout work during the holidays

• I am done painting the back of all rail meaning I could swap the layout around, so that the front is once again forward faced as intended. 
• I have replaced the wheels on the remaining two cars, so now all (3!) of them have wheels with proper /115 treads. 
• Finally I changed the wheel axles on the engine, so that my re-gauging for standard O gauge would work better. The existing axles were shorter and for the more narrow, but prototypically correct, Proto:48 gauge.

With all this done it was time for some running, But before I could do that I had to clean the rail heads (before I mess them up again when doing the weathering on the front side). Said and done, and with both clean engine wheels and rails I set out on a back and forth run. A few dirty spots remained, but with those taken care of I still had problems near then end of the track. In the very same spot the loco stopped dead every time.

A light push and it started up again, but then went silent. After many more attempts to clean rails and wheels, to no avail, I finally brought out my voltage meter. I soon found out that at this very spot none of the wheels on the right side made electrical contact with the rail beneath it! How could that be?

If I lightly pressed on the right of the loco there was once again contact. I first thought the problem was that the roadbed was leaning in some way, making the right hand wheels suspended in the air. But that could hardly be the case. The trucks would surely be able to take up any small (invisible) misalignment like that.

Something else must make the wheels rise. So what could make them climb? When I thought "climb" I finally understood. It turned out the track gauge was on the narrow side in that spot, which made the wheels climb ever so little. It was barely not visible, but enough to make the wheel treads loose contact with the rail heads. The flanges still had contact with the sides of the rail head, but there my newly applied paint and weathering stopped any electricity from coming through.

I really ought to pull the rail spikes in that section and move one of the rails, but for now I just removed the paint from the uppermost part of the rail head - and no more spurious stops in that spot.