Proto 48 is the most accurate O scale standard gauge model railroad modeling standard. Proto 48 follows exactly 1:48 SCALE. Gauge follows that scale exactly.
Therefore, standard gauge is not modeled at the usual 1.25 inches, but rather at 1.177 inches. The wheels are an exact AAR profile duplicating the prototype wheels. Instead of the usual oversized "O" scale "tire" and flange, the Proto 48 wheel is prototype 5.5 inches or 0.115" with a flange of 1 inch which is 0.02".Therefore, all wheels and axles need to be replaced.
This makes Proto 48 not compatible with any of the other "O" scale standards that use a prototype gauge of five feet which is to say 1.25 inches in "O" scale. The scale wheel shape also makes it not compatible with existing "O" track components like switches (points) of switch components like frogs, points, and guard rails. This means that all tracks must be hand-built and hand-laid.
Where Proto 48 IS compatible is with narrow gauges. Narrow gauge "O: scale has always been near prototype in wheel profile and the narrow gauges have always been correct. A three-foot gauge track measures an accurate 3/4 inch. A two-foot gauge track is an accurate 1/2 inch. A meter gauge track is an accurate 20.8mm. An On30 (On 2 1/2) (also known as Oe) is an accurate .625 inches (16.5mm)
https://proto48.org/