Ok boys and girls, here are the measurement results for the AM 27"R track and the Lionel 20"R FasTrack. First, AM markets 21"R, 24"R and 27"R curves. I have some of the 27"R curves that were included in their older sets. I have one Lionel set that included Fastrack. They market 20"R and 27"R.
The AM 27"R track makes a 54" diameter circle measured center to center. So it is correctly marketed to current track measurement standards.
The Lionel 20"R Fastrack makes a 38" diameter circle measured center to center. It is the same as the SHS/MTH track, it uses the old Gilbert protocol for radius measurement.
I do not have any 27"R FasTrack to measure, but I do have some of the old K-Line Gilbert style 27"R track. I have it in storage, when I find it I will post its dimensions. I do not have any American S gauge or GarGraves sectional track.