OO gauge or OO scale is the most popular standard gauge model railway standard in the United Kingdom, outside of which it is virtually unknown. OO gauge is one of several 4 mm-scale standards, and the only one to be marketed by major manufacturers. The OO track gauge of 16.5 mm corresponds to prototypical gauge of 4 ft 1+1⁄2 in, rather than 4 ft 8+1⁄2 in standard gauge. However, since the 1960s, other gauges in the same scale have arisen—18.2 mm (EM) and 18.83 mm (Scalefour)—to reflect the desire of some modellers for greater scale accuracy.
U.K. prototype model of a OO scale (1:76) British Rail Class 25 shown with five pence coin for scale
Hornby Railways Flying Scotsman locomotive on an OO gauge layout
Railway modelling or model railroading is a hobby in which rail transport systems are modelled at a reduced scale.
One of the smallest (Z scale, 1:220) placed on the buffer bar of one of the larger (live steam, 1:8) model locomotives
HO scale (1:87) model of a North American center cab switcher shown with a pencil for size
Z scale (1:220) scene of a 2-6-0 steam locomotive being turned. A scratch-built Russell snow plow is parked on a stub (Val Ease Central Railroad).
A simple H0 (1:87) scale model railroad, consisting of three interconnected modules, each 70 x 100 cm in size. It has two concentric ovals of track and a few switches to sidetracks. It makes no pretension of being a copy of "real life". Using low-cost landscaping parts, house kits and rolling stock, it was built for a total of only a few hundred dollars.
