Mark -- this topic has been discussed before, e.g., here: https://ogrforum.ogaugerr.com/...29#77593417171617329
I think the general consensus is that Lionel is both confused and confusing. Some of it appears to be trying to use a nomenclature they've used for O-gauge track - e.g. O-27, O-36, etc. So they've used S36 in some places, even though they don't have curved track with a 18" radius. Elsewhere they use R20, which is what most of us think of as the 'standard' curve track radius - dating back to the Flyer track made by Gilbert - which also applies to the Lionel reissue and the fast track. They do offer a larger, R27 curved track as well (I don't recall if they make it in fast track).
So the answer to your question is no, there is no difference. I wish Lionel would pay attention to this detail and not continue to be confused or confuse others...
