>Or perhaps it was other parts of the traction power system -

I recall some of the old heads talking about how the TMs were OK after conversion to a "transistorized throttle" but I never was able to determine what that meant.

The theory I put most stock in regarding F-M's demise in locomotive use is that in order to change a single piston (a "power assembly", so to speak), upper or lower, the entire upper crankshaft has to be removed. OTOH, F-M engines in submarines are better because if you're going to snorkel, the water in the tube has to be blown out the top, and with EMD engines this would occasionally blow the head off one of the cylinders. F-M engines weren't very susceptible to this.

>Since Grandpa had worked the electrical systems on Sacramento Northern's westinghouse equipped engines for many years,

Voltage-divider control power, pretty slick --- when it works.