O scale is a 1:48 proportion, or 1/4 inch to the foot.  O gauge track measures 1.26" between running rails.  This is the most popular toy train gauge by far.  The tinplate track is commonly found in two basic profiles, ragular O gauge and O-27, and at least five different curvatures.

 

Regular O gauge Lionel track has a railhead height of 1 1/16" and a circle diameter of 31".  Wider radius O-72 is of the same profile, but with a 72" diameter.

 

Track of the second profile, known as 0-27, is formed from lighter-weight steel and has a railhead height of 7/16".  A circle has a 27" diameter.  Two wider-radius versions of this track are also available from Lionel, 0-42 and 0-64.

 

The two profiles were not designed to be mixed.  They can be used together on the same layout, however, if the O-27 track is shimmed by 1/4" and the holes in the tubular rails are enlarged to accept regular O gauge pins.

 

Generally, most O gauge equipment, except for the largest locomotives and passenger cars, will operate satisfactorily on O-27 track.  And, with the exception of Lionel four-wheel-drive steamers made before 1948 and some early Marx units, most O-27 trains will work on regular O gauge track.

 

Several independent manufacturers also produce O gauge tinplate track in different curvatures, but in profiles similar to Lionel.