In On3, Sn3, HOn3, etc the "n" refers to narrow guage, and the 3 refers to 3 SCALE feet (American) between the rails, On2, Sn2, HOn2 would refer to 2 scale American feet between the rails, and On30, Sn30, and HOn30 would refer to 30 scale American inches (or 2.5 feet) between the rails. Most...