@MELGAR posted:"After reading this thread I checked one of my Atlas O NE-6 cabooses. First picture shows pitting on one of the thumbtacks that evidently had been making intermittent contact with the middle rail. The clearance between the thumbtack and rail was very small and slight downward pressure on the coupler caused it to contact the middle rail."
MELGAR
Until we saw the sparking when running in the dark, we had no idea the cause. Nice caboose.