... O27 track is lighter and the curves are sharper, making a 27" diameter circle. Hence the name "O27." ...
Rusty
O27 refers to the sharpest and most commonly available curves for the "lightweight" version of conventional 3-rail track. However it is also possible to get O42 and O54 curves in the "O27 profile" track, and some older compatible Marx track is "O34".
The sharpest curves in "Standard O" conventional 3-rail track are O31, but O42, O54 and O72 curves are also avaliable.
The two-digit numbers generally refer to the overall diameter of a circle of those track pieces, in inches.
In another post, I discuss use of different curve radii for good effect.