00 scale

OO gauge
U.K. prototype model of a 00 scale (1:76) British Rail Class 25 shown with 22.5 mm diameter Pound coin for scale
Scale	4 mm to 1ft
Scale ratio	1:76.2
Model gauge	16.5 mm (0.65 in)
Prototype gauge	Standard gauge

OO gauge or OO scale (also spelled 00 gauge and 00 scale) model railways are the most popular standard-gauge model railway tracks in the United Kingdom. This track gauge is one of several 4 mm-scale standards (4 mm to 1 foot or 1:76.2) used, but it is the only one to be served by the major manufacturers. Despite this, the OO track gauge of 16.5 mm (0.65 in) is inaccurate for 4 mm scale, and other gauges of the same scale have arisen to better serve the desires of some modellers for greater scale accuracy.

Double-0 scale model railways were launched by Bing in 1921 as 'The Table Railway', running on 16.5 mm (0.65 in) track and scaled at 4 mm to the foot. In 1922, the first models of British prototypes appeared. Initially all locomotives were powered by clockwork, but the first electric power appeared in autumn 1923.

OO describes models with a scale of 4 mm = 1 foot (1:76) running on HO scale 1:87 (3.5 mm = 1 foot) track (16.5 mm/0.650"). This combination came about as early clockwork mechanisms and electric motors were difficult to fit within HO scale models of British prototypes which are smaller than equivalent European and US locomotives. A quick and cheap solution was to enlarge the scale of the model to 4 mm to the foot but keep the 3.5 mm to the foot gauge track. This also allowed more space to model the external valve gear. The resulting HO track gauge of 16.5 mm represents 4 feet 1.5 inches at 4 mm to the foot scale, this is 7 inches under scale or is approximately 2.33 mm too narrow.

In 1932 the Bing company collapsed, but the Table Railway continued to be manufactured by the new Trix company. Trix decided to use the new HO standard, being approximately half of European 0 gauge (1:43 scale).

In 1938, the Meccano Company launched a new range of OO models under the trade name of Hornby Dublo; OO gauge has remained the UK's most popular scale/gauge ever since.

In the United States, Lionel Corporation introduced a range of OO models in 1938. Soon other companies followed but it did not prove popular and remained on the market only until 1942. OO gauge was quickly eclipsed by HO scale. The Lionel range of OO used 19 mm/¾" track gauge, a scale 57", a track width that was more to scale. There is a small following of American OO scale/gauge today.