Originally Posted by
lincolnjkc
I can't figure out where QXA is/was but it doesn't appear to be a valid IATA city code

Probably a special system code and not a location - FAA LIDs don't start with Q and legacy NW probably wasn't using a real, non-US location. Q codes are sometimes used these days to represent "any airport in a location" that don't have a formally recognized one like CHI or NYC, e.g. QSF or QLA are supported by some systems.