Welcome to the On30 Page. On30, what does this mean. "O" indicates we are modeling in O-scale which in North America is a ratio of 1 to 48 or 1:78 or 1 foot in the model equals 48 feet in the real world. O-scale trains run on 9mm track. Standard gauge track in the real world is 4ft 8.5 inches between the rails. The second letter, "n" indicates that instead of standard gauge we are modeling narrow gauge. This could be anything from 18 inches between the rails to 42 inches between the rails. "30" indicates that in fact we are modeling in O-scale railroads that ran on track which was 30 inches between the rails.
So much for the technical side. In practice we use wheels and track that was developed for HO-gauge trains to run on. This equates to 30 inches between the rails in real life.