Discovered an interesting problem with Gargraves (and maybe other types as well) uncouplers.
I had them wired as instructed, with the magnet lead "hot" and the circuit completed pressing the button on the "common" leg.
A caboose with the under-truck plate for activitating the coupler touched the magnet when activated and, making it's own "ground" welded itself to the magnet!!!
Reversed the wiring so the magnet lead is always "cold" until the button is pressed, sending current to it.
When the caboose armature touched the magnet this time, I released the button and the circuit was broken.
Wired all the rest that way and end of problem.