@gunrunnerjohn posted:Stan, I'm having real difficulty with the concept of a car that would have a problem only switching one side of the power. I can't imagine the need for it, and I don't recall ever seeing one that would require that.
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Right. This is one of those ask me how I know. 
In some MTH operating cars, power can be provided using the normal truck roller-wheels (i.e., center-outer rails)...OR by the 4th/5th rails of the UCS. Of course the 4th/5th rails are only active when the UNLOAD button is pressed. The UNLOAD is a double-pole switch that applies track power to the 4th and 5th rails. The rub is an operating car can be facing either direction on the UCS. So it's a coin toss as to whether the 4th rail sliding shoe receives hot or common. Hence you cannot pre-wire one side and use only a single-pole switch (relay) to apply AC such cars.
I know the FET version will work on the MTH operating boxcars. It remains to be seen if the caboose's conventional bell DC offset detector (which the boxcar does not have) precludes a simple 3-wire connection of the FET version.
