Mark,
The other track with 4-rails is indeed for Gauge 1 (the outer 2-rail) and for Gauge 0 (since the rails are not electrically interconnected I can run 2-rail and 3-rail 0 Gauge). See the picture:
And on the Gauge 1 track with the extra rail for S Gauge I can also run 3-rail electric Gauge 1.
I do not have any prewar 3-rail 0 gauge American Flyer (I have never seen it over-here even) but I could stage a race between Flyer and Lionel
Regards
Fred