doing a little trig.... take a straight edge or a ruler greater than 14" and lay it down as a chord between the end points of the center rail on a single piece of track. at the center of that chord, measure the distance from the straight edge to the center rail. if it is O72 (16 pieces to a circle) that measurement will be 0.70". if the track segment is 1/12 of an O54 circle, that measurement will be closer to 0.90"
unfortunately just measuring the chord on an O72 (16 piece circle) or an O54 (12 piece circle), they turn out to be pretty darn close to equal. a 22.5° chord on a 36" radius circle is 14.04" and a 30° chord on a 27" radius circle is 13.96". too close to differentiate in measurement with toy train accuracy.
math is fun!
[edit... corrected some math]