OO gauge

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OO gauge
UK prototype model of a 00 scale (1:76) British Rail Class 25 locomotive shown with an 18 millimetres (0.7 inches) diameter five pence coin
Scale	4 mm to 1 ft
Scale ratio	1:76.2
Model gauge	16.5 mm (0.65 in)
Prototype gauge	Standard gauge

The terms OO gauge and OO scale (or more correctly but less commonly, 00 gauge and 00 scale) relate to the most popular standard gauge model railway standard in the United Kingdom,^[1] outside of which it is virtually unknown. "00" is a variant of "H0", meaning Half-0, which historically derives (in increasing size order) from 0 scale, 1 scale and 2 scale, the most popular scales in the early 20th century. Since railway modellers invariably pronounce the zero as "oh" rather than "zero" (e.g. "double-oh" or "aitch-oh"), the scales are often written as OO, HO and O.

00 scale is one of several 4 mm-scale standards (4 mm to the foot or 1:76.2), and the only one to be marketed by major manufacturers of British-outline models.

Logically, to replicate the full-size ("prototype") standard gauge of 1435 mm (4 ft 8+1⁄2 in) the track gauge at 4 mm-to-the-foot scale would be 18.83 millimetres (0.741 inches). However, the gauge is 16.5 mm (0.65 in), which is the same as in H0 scale – 3.5 mm to the foot or 1:87. This oddity has historical origins: essentially, 00 scale involves 4 mm-to-the-foot bodies being mounted on 3.5 mm-to-the-foot track. The result is that 00 rolling stock appears to be running on narrow gauge. The anomaly led some 4 scale modellers in the 1960s to adopt a gauge of 18.2 mm (EM scale), soon followed by some who decided to adopt 18.83 mm and wheel/track proportions very close to full-scale practice (Protofour standards).^[2][3]

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 1923.

"00" 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 in).^[4] This combination came about since early clockwork mechanisms and electric motors were difficult to fit within H0 scale models of British trains, which have a smaller loading gauge than their European and North American counterparts and locomotives with smaller, often tapered, boilers. A 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, which is 7 inches too narrow or approximately 2.33 mm too narrow in the model.

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 half of French 0 gauge (1:43.5 scale). However, European 0 scale is 1:45.

In 1938, the Meccano Company launched a new range of 00 models under the trade name of Hornby Dublo. The combination of 4 mm scale and 16.5 mm gauge has remained the UK's most popular scale and gauge ever since.

In the United States, Lionel Corporation introduced a range of 00 models in 1938. Soon other companies followed but it did not prove popular and remained on the market only until 1942, when Lionel train production was shut down due to wartime restrictions to the use of steel. 00 was quickly eclipsed by the better-proportioned H0 scale. The Lionel range of 00 used 19 mm (3⁄4 inch) track gauge, equating to 57 inches or 4 ft 9 in – very close to the 4 ft 81⁄2 in of standard gauge. The following of American 00 scale declined and today it is negligible.^{[citation needed]}

00 remains the most popular scale for railway modelling in Great Britain due to a ready availability of ready-to-run rolling stock and starter sets. Ready-to-run in the UK is dominated by Hornby Railways and Bachmann Branchline.^[5][6] Other sources of ready-to-run rolling stock or locomotives include Dapol, Heljan, Peco, ViTrains, Rapido Trains UK, Accurascale, and previously Lima, Tri-ang Railways, and Mainline Railways. Other scales, with the possible exception of N , lack the variety and affordability of UK ready-to-run products.^{[citation needed]}

16.5 mm (0.65 in) gauge at 4 mm-to-the-footfoot means that the scale gauge represents 4 ft 1+1⁄2 in (1257 mm), 7 inches (178 mm) narrower than the prototype 4 ft 8+1⁄2 in (1435 mm). This difference is particularly noticeable when looking along the track.

Some modellers use 00 track to represent the 5 ft 3 in (1600 mm) Irish gauge, where it is a scale 13+1⁄2 inches (343 mm) too narrow.

These differences have led to the development of the finescale standards of EM gauge and Protofour.

In common with most practical model railways of any scale (and not related to the 00 gauge inaccuracy) the following compromises are made:

• Curves are usually much sharper than the prototype, and often not transitioned, particularly when using "set-track" systems (radius 1 = 371 mm, 2 = 438 mm, 3 = 505 mm, 4 = 571.5 mm).
• Overhang from long vehicles necessitates the track centres being increased to prevent collisions on curves between rolling stock on adjacent lines.
• Over-scale wheel width and flange depth are used on typical models (particularly older models), which require over-scale rail profiles and much larger clearances on pointwork than is prototypical. Pointwork is often compressed in length to save space.

Many modellers after starting in 00 scale find its "narrow-gauge" appearance unsatisfactory. Greater accuracy is possible using EM or the closer-to-exact-scale P4 track. Products and information are supplied by the The EM Gauge Society and Scalefour Society respectively.

• Rail transport modelling scales

• OO9 – Used for modelling 2 ft (610 mm) narrow gauge railways in 4 mm scale, often paired with OO scale layouts.
• 00n3 – Used for modelling 3 ft (914 mm) narrow gauge railways in 4 mm scale with 12 mm (0.472 in) gauge track.
• H0 – 3.5 mm scale using the same 16.5 mm (0.65 in) gauge track as 00.
• EM – 4 mm scale using 18.2 mm (0.717 in) track.
• P4 – A set of 4 mm standards using 18.83 mm (0.741 in) gauge track (accurate scale standard gauge-track).
• 00-SF – Uses 16.2 mm (0.638 in) track with ordinary 00 wheelsets. Allows the tighter trackwork tolerances of EM without the need to re-gauge wheels.

• Accurascale – A relatively new manufacturer founded in 2015 producing UK and Irish locomotives and rolling stock.^[7]
• Airfix – Bought the Kitmaster range and sold it under the Airfix brand until the original Airfix company collapsed in 1981. Some of the tooling was then destroyed, but Dapol bought the remainder. Most Airfix military vehicles are also to 1:76 scale.
• Bachmann Branchline – One of the largest manufacturers of ready-to-run 00.
• Dapol – Produce kits (using the Kitmaster toolings bought from Airfix) and ready-to-run locomotives and rolling stock.
• Hattons Model Railways – A former model railway retailer that also produced ready to run 00 models. The model toolings were sold to Rails of Sheffield and Accurascale when the company ceased trading in 2024.^[8]
• Heljan – A semi-major player in the business, less notable than other companies though.
• Hornby – One of the largest manufacturers of ready-to-run 00. Originally known as 'Hornby Railways'.
• Kitmaster – Manufactured plastic model kits of locomotives, rolling stock, and buildings.
• Lima – Produced budget 00 ready-to-run, bought by Hornby.
• Peco – Produce a wide range of track, kits, and other accessories.
• Rapido Trains UK – A subsidiary brand of the Canadian Rapido Trains Inc. specialising in UK model trains.^[9]
• Sonic Models – A relatively new manufacturer that makes 00 and N scale models.^[10]
• Tri-ang Railways – A former manufacturer whose name was dropped when their parent company acquired the Hornby brand; made models of locomotives and rolling stock.
• Planet Industrials – A new manufacturer in the business, known mostly for kits but has produced Ready to Run (RTR) versions of the Kerr, Stuart and Company 'Victory' class locomotive.^[11]

Wikimedia Commons has media related to 00 scale.

• Comprehensive history of the development of model railway scales, especially 00.
• The Double O Gauge Association
• The EM Gauge Society
• The Scalefour Society