Well I didn't think that all the way through, did I? In which case, I second the ideas about shorts caused by backwards wheel-sets or brake shoes etc.
But at the risk of losing any last shred of credibility, I'll take one more stab at it: Could the motors (if still present) possibly have capacitors across their terminals for RF suppression? I have a Sunset-3rd Rail non-DCC 2-rail CPR D10 which caused an increasing hum for a few seconds followed by booster shut-down when I first put it on the track. I looked all over for shorts and found none, but in the end it was caused by a capacitor across the motor.