Welcome to the Nn3 Page. Nn3, what does this mean. "N" indicates we are modeling in N-scale which in North America is a ratio of 1 to 160 or 1:160 or 1 foot in the model equals 160 feet in the real world. N-scale trains run on 9mm track. Standard gauge track in the real world is 4ft 8.5 inches between the rails. So in N-Scale standard gauge track is represented by 9mm track or 9mm between the rails. The second letter, "n" indicates that instead of standard gauge we are modeling narrow gauge. The could be anything from 18 inches between the rails to 42 inches between the rails. "3" indicates that in fact we are modeling in N-scale railroads that ran on track which was 3 feet between the rails.
So much for the technical side. In practice we use track that was developed for Z-gauge trains to run on and it is 6.5mm between the rails. This equates to 42 inch or 3ft 6in between the rail in real life. However in practical terms we model Meter gauge, 3 ft gauge and 3ft 6in gauge in N-scale using 6.5mm track.