HO is the most popular scale with the greatest selection of trains and accessories, as it allows lots of railroad action in a small area. What does HO stand for? The designation "HO" means "Half of O scale". These models are half the size of O Scale. Their exact scale is 1:87 or 1/87th the actual size of the prototype (1 foot equals 87 feet). The gauge of HO track is 16.5mm between the rails. Children may need adult help to set up or take down train sets.