Traditionally, American OO gauge is either described as 19mm or ¾”. For the purposes of those working in OO back in the day, that was practically the same dimension, as .750 inch is 19.05mm. Or, going the other direction, 19mm is 0.748031 inch. With 4mm being 1 foot, 19mm gauge track is 4’9” gauge, only a half inch over. It was good enough for the pioneers of the scale, and an easy dimension to measure.

So, what did manufacturers actually do back in the day? Wheelsets could be compared, but that is not reliable, as they were not necessarily to gauge, and also could have moved over time. More reliably, vintage track gauges or sectional track can be used. Of the two lines of sectional track, comparing sections of straight track with calipers, Scale-Craft is a bit under .750 (3/4″), and Lionel 3-rail straight track is gauged at about .760. This makes some sense, as Lionel wheelset treads are much wider, with deep flanges, when compared to Scale-Craft or Nason.

So, again, ¾” or 19mm is recognized as being American OO gauge. But with that said, periodically people have advocated for using a slightly smaller, corrected gauge. In the UK presently there are two versions of this in use, S4 (Scalefour) and P4 (Protofour). From the Scalefour Society website, “…we believe that finescale standards begin with wheels and track, and for that reason the Scalefour Society promotes the P4 standards that produce trackwork to an accurate standard gauge of 18.83mm [0.74133858”], and use correctly-profiled wheels.”

This is where problems begin for the American OO operator or collectors of today. I do not believe traditional American OO will operate on the UK S4 or P4 track unless the wheelsets are modified or replaced. I suppose you could come up with a compromise version with NMRA wheel standards at 18.83mm gauge. But with the current NMRA standards (for OO and On3 — the standards are the same), Lionel equipment operates OK. I do run it often on my layout. Their flanges are so heavy compared to current flange standards that they will bump over my turnouts, and can hit spike heads if you use code 100 rail like I do. But the important thing is that Lionel OO will operate on track built to the current standards, which would be a must for any new standards that might be created.

(Not to mention future track products! There is a huge need for track today, and Lionel OO must operate well on any track that might be produced.)

Personally, while I understand the idea of being more correct, I would advocate for sticking with the current NMRA OO standards. The NMRA standards gauge for OO/On3 is an indispensable tool, and I have no interest in rebuilding things to a narrower gauge or with fine scale standards. This would involve modifications of basically all drivers and wheels. As a retro modeler, count me as being against this idea. But if it excites you, by all means adopt something like P4 or S4 for your layout! As has been proven in the UK, there is room for multiple “flavors” of OO gauge.