Artie--I am doing my best not to remove the track. Since this problem only affects a small percentage of my wiring, I am going to trust the screw attachment. If I have to lift the track section, I might as well replace it with a known-good track section.
GRJ and friends--it is quite possible that some stainless rail is mixed in here. I bought almost all my track USED, like everything else on this railroad! So Lord knows what is in there. I guess I am lucky that the problem is not more widespread.
Fastman--that magnet idea is worth trying and I will do that ASAP.
Don