I finally solved the K-Line uncoupling track mystery! The hot lead was grounded to the outer rail. This is probably because it was previously used on a system where the outer rail was hot? I have to decide who to splice into this wire. On future tracks of this kind, I can get the ground off the screw on the magnet rather than straight from the outer rail. I still want to experiment with a constant voltage source for the uncoupling magnet.