HO gauge ( Half O ) is the standard size choice made for many indoor layouts when space constraints dictate against using O or a larger gauge such as G (and are not so severe as to compel a switch to N or smaller). HO is sometimes referred to as 3.5mm gauge and typically measures 16mm between the rails.
RR-Track currently has 15 HO-gauge track libraries available for the layout designer. Nine of these are two-rail and somewhat mutually compatible, while the other three are AC three-rail Märklin and compatible only with each other. Click on any of the libraries at left to display the contents.