Tom -- nice job! So SHS (and now MTH) seems to use the sensible labeling of track radius to the centerline (well, except for their confusion about the two labels for the same track). Just for grins I looked on Doug Peck's Portlines site and here are the numbers used for other S-gauge track makers:
Gargraves (labeled as diameters): 42, 54, 63 and 72", where 8 sections make a circle
American Models (radii): 21, 24, 27" (12 sections to a circle)
American S-Gauge track (radii): 21, 27" (12 sections to a circle
Flyonel Fastrack (radius): 20" (12 sections to a circle) -- (I thought there was a 27" radius track also?)
So clearly we need more measurement data!
- Rich