While rail sizes today are commonly referred to by “code” (which I will come back to) prior to that the NMRA set a series of alphabetical standards for rails. An article in the December, 1945 issue of Model Railroader reviews these early standards. Titled “N.M.R.A. Engineering Committee Defines Standard Scale Model Rail Sections” the article notes that in the prewar era

…there were some 50 different model rail sections manufactured. In the interests of standardization and production economy the N.M.R.A. suggested that rail nomenclature be changed so that there would be a single series of model rail sections running from the largest to the smallest, and that the table be published correlating these sizes with correct scale in each gauge. In the smaller gauges where even the smallest rail was oversize for the largest common prototype section of 132 lbs., footnotes to the table designated that certain sections were commonly used in these gauges.

The table shows the following actual sizes:

Section Code Height
A .245”
B .172”
C .158”
D .125”
E .115”
F .100”

The letters are not used today but instead we refer to the height as a “code.” For example, code 100 rail is the standard used the common brands of HO track with representing a rail size of approximately 152 lbs which would be the largest prototype size and was only rarely used in reality. Also note in 1945 there were apparently no rails smaller than .100” manufactured.

Back to OO, the sizes that would be used in this system are D, E, and F. A lot of OO rail back in the day was similar to this D section which was close to the 152 lbs type of rail. See for example this photo, which is a close up of the rail supplied on Scale-Craft sectional track compared to a modern code 100 rail. The rail section used by S-C was overly wide, as explained by the below.

On the standards ballot this year is a proposal to set up a new section corresponding very closely to the present D section except that the head will be substantially narrower making a rail more truly in scale proportions to prototype running rails.

The present D section was originally intended as a third rail for O gauge but has been widely used for running rail in OO gauge. The new rail will be equally satisfactory and of better appearance in OO gauge….

The D section was noted in the chart as being “commonly used in OO.” But OO gaugers themselves were moving toward the F/code 100 rail section, which was similar to the rail used by Mantua and Midlin for their OO track products and represented a very prototypical 132 lbs. rail type typical of heavy mainline rail. I use code 100 on my layout; it works great for everything, even Lionel if you take care to get the spike heads down well.