American Flyer produced track in a variety of gauges and radii, but if you’re asking about American Flyer track radius, you probably mean its S gauge track.
The standard radius for American Flyer S gauge track, produced from 1946 to 1967, was 20 inches. Other radii were also available from other manufacturers.
Standard American Flyer track
Standard American Flyer S gauge track had a diameter of 40 inches and a radius of 20 inches. This provided smooth operation for S scale steam engines, comparable to how HO scale engines run on 18-inch radius track. It permitted greater detail and smoother operation than its main competitor, Lionel, who ran larger trains on smaller, 15.5-inch radius track.
This track radius provided a competitive advantage on paper, but some difficulties in practice. The 20-inch radius didn’t provide a lot of flexibility for a layout on a 4×8 sheet of plywood. If you wanted to do something more than a basic oval, 5×9 worked better. This was fine if you repurposed a ping pong table for a layout, but if you were building from standard lumber, 4×8 sheets have always been much easier to find.
Lionel’s S gauge Fastrack is available in a 20-inch radius, and has transition pieces to allow interoperability with vintage American Flyer track.
American Flyer Pikemaster track
In the 1960s, Gilbert experimented with a new type of track it called Pikemaster. It used shorter, more realistic rails and plastic ties with realistic spacing, looking a lot like the HO scale track of the time. It also featured a smaller radius, of about 15.5 inches, or a 31-inch diameter, matching Lionel. This made smaller layouts much more practical, though larger engines had a harder time negotiating the track. The changes rendered existing track locks, terminals, and connectors incompatible.
Gilbert didn’t discontinue the old track, but it bundled Pikemaster track in all of its new sets.
Pikemaster track looked good but wasn’t as durable or reliable, and didn’t operate as smoothly. Consumer Reports reviewed it in 1962 and deemed it flimsy, and deemed the switches unacceptable. Flyer fans today associate Pikemaster with Gilbert’s decline and eventual bankruptcy.
27″ radius American Flyer track
Wider-radius American Flyer track came about in the 1990s thanks to K-Line. This track was a wider radius, 27 inches, or 54 inches in diameter, for smoother running. It was the same profile as Flyer postwar track, so it was completely compatible with the earlier, classic pre-Pikemaster design.
After K-Line went out of business, Lionel acquired the tooling for this track and continued production for several years. So there is American Flyer-branded 27″-radius track in existence, but it wasn’t produced by Gilbert.
Lionel has phased this track out in favor of Fastrack, but does offer Fastrack in the wider 27-inch radius.
O and Standard Gauge
Prior to World War II, American Flyer made O gauge and Standard Gauge trains. Early Coleman-era track followed Lionel’s standards of 27-inch and 31-inch diameters (13.5 and 15.5-inch radius) for O gauge, and 42 inches diameter (21-inch radius) for Standard gauge.
After Gilbert bought American Flyer, he changed to an unusual 40-inch diameter (20-inch radius), which matched what he used for the postwar S gauge design.