You did say originally that it happened in same spots. "Every time they go around the rail they will do this same thing at the same spots."
A steam loco is too long to run the conventional test. Tender wheels could bridge a ground gap. My layout had been up for years with no problems, until I ran a deadhead Docksider and found two very short groundless spots.
Reason I asked about a trailing truck is that some locos, with neither trailing trucks nor axle wipers, have experienced ground issues.
Are you sure rolling stock isn't fouling on something?
