ON30 and ON3 are both valid track gauges. However, very little common carrier, if any, was built to 30 inch gauge in the USA. I think there has been quite a bit of industrial narrow gauge at 30 inches. 3 Foot gauge (36 inches) was used a lot for common carrier shortlines in the 1870s and so. There was a major movement to build "narrow" gauge lines because they were cheaper to build and maintain in every way. Colorado had over 1700 miles of narrow gauge at one time. The East Broad Top in Pennsylvania is another famous 3 ft line. There was also the Ohio River and Western which ran from near Wheeling to Zanesville OH and the Washington and Waynesville which ran between the namesake towns. The B&O was involved in the Pittsburgh Southern which was built to 3 ft and then standard gauged in a few years. Maine had a small system of 2 ft gauge common carriers.
so could you use ON30 track to model ON3? Probably that would work as well as using 5 ft gauge to model 4 ft 8 1/2. Some people will not like it and some wouldn't do it.
