| OO | |
| U.K. prototype model of a 00 scale (1:76) British Rail Class 25 shown with an 18mm-Five Pence coin for scale | |
| Scale per foot: | 4 mm to 1ft |
| Scale ratio: | 1:76.2 |
| Gauge: | 16.5 mm (0.650 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 U.K. This track gauge is one of several 4mm-scale standards (4 mm:foot / 304.8 mm 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 (0.650 in) is inaccurate for 4mm scale, and other gauges of the same scale have arisen to better serve the desires of some modellers for greater scale accuracy.
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Double-0 scale model railways were launched by Bing in 1921 as 'The Table Railway', running on 16.5 mm (0.650 in) track and scaled at 4mm 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 4mm = 1 foot (1:76) running on HO scale 1:87 (3.5mm = 1 foot) track (16.5mm/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 4mm to the foot but keep the 3.5mm to the foot gauge track. This also allowed more space to model the external valve gear. The resulting HO track gauge of 16.5mm represents 4 feet 1.5 inches at 4mm to the foot scale, this is 7 inches under scale or is approximately 2.33mm 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 O 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 19mm/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.
16.5 mm (0.650 in) gauge at 4 mm:1-foot means that the scale gauge represents 4 ft 1 1⁄2 in (1,257 mm), 7 inches (178 mm) narrower than the prototype 4 ft 8 1⁄2 in (1,435 mm). This noticeable difference are aggravated if over-scale rail section, over-scale wheel width and deep wheel flanges are used on typical models. These departures from scale require much larger clearances on pointwork and are particularly noticeable when looking along the track. This scale gauge more accurately represents the narrow gauge railways built to 4 ft (1,219 mm) gauge, for example the Padarn Railway and Saundersfoot Railway in Wales and the Glasgow Subway in Scotland.
Though they run on the same track, OO gauge and HO gauge models of the same prototype do not sit well together since the OO models are larger than the HO equivalent.
OO is also used to represent the 5 ft 3 in (1,600 mm) Irish gauge, where it is a scale 131⁄2 inches (343 mm) too narrow.
These differences have led to the development of the finescale standards EM gauge and P4 scale.
OO remains the most popular scale for railway modelling in the United Kingdom; this is most likely attributable to a ready availability of ready-to-run stock and starter sets. Ready-to-run in the UK is dominated by Hornby Railways and Bachmann Branchline. Other sources of ready-to-run rolling stock or locomotives include the Dapol, Heljan, Peco, ViTrains and previously Lima. Other scales, with the possible exception of N gauge, lack the variety and affordability of UK ready-to-run products, it is likely that this deters British modellers and leads to the prevalence of OO.
Good results in OO can be achieved despite the scale inaccuracies with modern ready-to-run equipment on ballasted Code 75 trackwork, with realistic track spacing (the "6-foot"), and try to minimise or hide tight curves where necessary.
Many experienced modellers find the OO standard produces a "narrow gauge" appearance when the model is viewed from head on. Greater accuracy is possible using either EM gauge or the closer to exact scale P4 gauge track.
Whilst flextrack is available for both EM and P4 gauges (from manufacturers such as C&L Finescale, SMP and The P4 Track Company), ready-to-run (RTR) point and crossing (P&C) work is not available, so this trackwork must be constructed by the modeller. Kits for doing this are also available from the aforementioned sources amongst others. Several of these kits are also available for the OO modeller who aims for more realistic track, since most RTR track is actually scaled to HO and does not represent any British prototype and the sleeper spacing is too close together for scale. EM gauge has slightly overscale flanges and flangeways on point and crossing work; P4 is closer to scale but the smaller flanges and flangeways on P&C work expose poor track construction.