PRRJIM - yes but. The difference here is in the labeling used, not in the actual diameter - as you will see in the discussion in this thread and in the linked thread. Flyer did not and Lionel does not make two different S gauge curved tracks with radius/diameter of 18/36" and 20/40" (they did and still do make a larger radius/diameter curve of 27/54"). So the issue here is the inconsistency in labeling by Lionel as applied to exactly the same radius/diameter curved track. In their catalogs, Lionel says "minimum curve: S36" and "minimum curve: R20" seemingly interchangeably - for example, for the EP-5 (p. 109 of the current catalog), it sez R20 while for the U36C on p. 108, it sez S36. Nevermind the fact that the wheelbase for the EP-5 is shorter than that of the U36C... BTW, the S gauge fast track sold by Lionel is clearly labeled R20 and R27...
I suppose, trying to give Lionel some credit here , maybe they are trying to say that if you are using flexible track, you shouldn't make the curves tighter than S36 (R18) or R20. Of course, that seems completely inconsistent with the EP-5 vs U36C comparison above...
At the end of the day, I think Rusty has it right