I bought some Fama track (since S gauge track was not available where I live) and tried tried to run American Flyer S gauge trains on Fama track. I found out the wheels touch the rail-chairs. After removing some plastic of the rails chairs it worked, but that is of course not a nice solution; but it did work:
So Fama and American Flyer are not compatible. I assume more scale like S gauge trains can run on Fama track.
Regards
Fred
BTW: Maybe this question could better be shifted to the S forum: https://ogrforum.ogaugerr.com/forum/s-scale-trains