Damage Control

I would say use DC all the time, and if you have any equipment that can't tolerate DC, keep it off the turntable whisker track. You could install an normally OFF momentary interlock switch, that keeps power to the first foot or so of the whisker track interrupted, until needed,. It will remind you to keep non-DC trains off the siding.

Arthur, I can see your brain gears were turning on this. Thanks for taking the time to understand what I'm talking about. Another aspect of this is my milk car unloader is on the whistler track.  I don't think the unloading/uncoupling track can tolerate DC or the milk car itself.  Also, other "automated" cars are not likely to tolerate DC.  I'm going to switch the unloader/uncoupler track OFF via the relay when on DC

These magnets and solenoids are designed for AC and have a certain impedance (AC resistance) to limit the coil's current while maximizing magnetization.  They will tolerate DC for short (few seconds?) periods but the DC current is limited only by wire resistance not AC impedance, thus these coils run on DC for too long a period will overheat, leading to "magic smoke" appearance.