On the real railroads as well as toy trains, gauge refers to the distance between the outside rails of the track.
Lionel O gauge track systems—which include O, O-27, and FasTrack®—all measure 1 ¼” between the outer rails. Any O gauge car or locomotive can ride on Lionel O gauge track, as long as the curve track used on your layout can accommodate your particular cars (see Minimum Curve section).
Lionel S gauge track (AF FasTrack) is narrower than O, and supports American Flyer and other S gauge cars.
