American Flyer radius is 20 inches, what is the radius of Lionel 3 rail? that is what needs to be compared, not Lionel 3 rail to S scale. How about Lionel 3 rail to O scale. Do they all operate on the same radius? it's not the radius as much as the design of the model. this argument has been around forever and it all depends on how eyes are looking at it. American Flyer fans wit tell you they have real scale trains. Yeah, they have two rail AC power and the Lionel has 3 rail AC power.
Traditionally, Lionel track is measured in diameter, not radius. O27 track has a diameter of 27" outer rail to outer rail. O31 is 31" outer rail to outer rail. That makes the radii approximately 13" and 15" respectively. Traditional Flyer track is 40" (give or take) outer rail to outer rail.
So, in reality in olden days, one could build a Lionel layout in less space than a Flyer layout.
I think nowadays the O Gauge gang still use diameters for the newer, larger curves, but measured from the center rail.
The other Rusty