Scale is the model scaled down in size. i.e. 1" in a scale model = 1 foot in real size Gauge is the size of the track, the distance between the two rails. HO and OO scale trains both run on OO gauge rails. HO is the true scale for the track as OO is slightly bigger to accommodate the larger Hornby motors when they first made them many years ago. OO can also be run on N guage if converted, so they then become narrow gauge railway. The OO guage comes from the Hornby name Dublo which was shorten to OO.