ERR and postwar F-3

Easy to determine, break out your ohmmeter and check the two pins to ground.  You'll find it's always the one closest to the end of the board.

Note that the fact that it's on the end means depending on the front or rear coupler, a different pin will be ground and hot on the plug-in connector.

Not train info specifically, but hot or common may be the actual wire turned off or on for any circuit. More modern stuff I’ve encountered adds or removes the common/ground side more often than hot.  Testing the leads for voltage to board ground tells me if wire #A is a switched hot. Testing the leads for continuity to board ground tells me if wire #A is a switched ground. (usually anyhow).  Getting it wrong short term tests rarely cook something else (never for me so far)

Trust me, the "ground" pins are really frame ground on the ERR boards, they don't do anything but provide ground potential. 

Thanks I will break out the meter

In this case Art, I gave you the answer, but verifying stuff like this with a meter is never a bad idea.