Bill,
I have never worked on a 76, so I can't give a specific fix.
1. I've worked on Marx bell ringing signals, and Marx completed the ground side of the circuit through a contact attached to the striker arm that allowed the circuit to be made and broken between pulls of the coil which caused the bell to ring continuously due to the 60 cycle alternation.
2. My other guess would be that Lionel might have used a thermostatic strip as they did on the bell ringing tenders.
In either case, dirty or oxidized contacts will cause trouble.