Allan, I can recall seeing the wreck of the 3rd unit at Edaville. Unfortunately I can't recall what year that was, but it was down where the new Edaville engine shop is, out back. That move from Portland back to Edaville may be when MNG started to move surplus equipment from their site.
I have two photos of Sonny Fairbanks handcar line that were just given to me by Allan Socea. I had estimated the track distance at 300', so that dovetails in with what you found when you worked there. The Socea collection photos are from 1936 and '37 and in one, Sonny is looking like he's having a lot of fun. The second shot shows his dog on the handcar looking like he'd rather be doing dog things.