Pulled out all 3 of my tunnel motors. KCS and SP from the 2005 run and the Cotton Belt Bicentennial unit from the 2009 run (Dates according to Trovestar database). All 3 had never been on the layout. Didn't pull the shells yet but the KCS and SP did run a few test laps just fine. The Cotton Belt Bicentennial unit and the motor runs but the unit doesn't move. Acts like one of the Bachmann units with the split gears. Before I pull the shell and make a mess, could this be due to zinc rot or does it sound like something else?
It seems that you have to bite the bullet and pull the shell to find out. Sounds like there is a problem in the drive train. Until you can see it all, one can only speculate.