On30 uses the American O scale of 1/4" to the foot, (ratio 1:48) to operate trains on HO gauge (16.5 mm/0.650 in) track. The 30 indicates the scale/gauge combination is used to model 2 ft 6 in (762 mm) (30") narrow gauge prototypes, although it is often used to model 2 ft (610 mm) and 3 ft...