I think, for now, that tubular track is best classified into the two height categories but, unfortunately, one of the height categories has been named/labeled based on a long-ago constraint of only being available in 27" diameter curves. Hence, based on tradition, we're stuck with O and O27 for the high and low profile tubular track types.
I inherited O27 sets, track, and PW accessories but very quickly realized the benefit of going with wider-diameter curves. I own 34", 42", 54" and, yes, 72" curves in lower O27 profile. I believe Marx was the only one to make 34", several made 42", and K-Line solely made 54" and 72".
I don't have data on O, but am pretty sure it's been offered in 31", 42", 54" and 72", maybe more.
