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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.

OO gauge
OO Scale BR 25054.JPG
U.K. prototype model of a 00 scale (1:76) British Rail Class 25 shown with five pence 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



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 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.

OO todayEdit

OO remains the most popular scale for railway modelling in the Great Britain due 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 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. The quality of OO models has improved over time.

Scaling and accuracyEdit

16.5 mm (0.65 in) gauge at 4 mm:1-foot means that the scale gauge represents 4 ft 1 12 in (1,257 mm), 7 inches (178 mm) narrower than the prototype 4 ft 8 12 in (1,435 mm). This difference is particularly noticeable when looking along the track. As the market for proprietary track is mostly for HO scale, sleeper size and spacing are designed for HO and are therefore underscale.

OO is also used to represent the 5 ft 3 in (1,600 mm) Irish gauge, where it is a scale 13 12 inches (343 mm) too narrow.

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.

These differences have led to the development of the finescale standards of EM gauge and P4 standards. Nevertheless, it is possible to model using OO to standards that fall just short of finescale.

In common with most practical model railways of any scale (and not related to the OO gauge inaccuracy) the following compromises are made: Curves are often 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 means that track centres are overscale to prevent collisions on curves between stock on adjacent lines, at up to 65 mm (for set-track (reduced down to 50 mm for Peco Streamline)). Overscale wheel width and deep wheel flanges are used on typical models (but particularly older models), and these require overscale rail profile and much larger clearances on pointwork than is prototypical. Pointwork is often compressed in length to save space.

4 mm finescale standardsEdit

Many experienced modellers[citation needed] 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 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.

See alsoEdit

Other model railway scalesEdit

Related scalesEdit

  • OO9 – Used for modelling 2 ft (610 mm) narrow gauge railways in 4 mm scale
  • OOn3 – Used for modelling 3 ft (914 mm) narrow gauge railways in 4 mm scale
  • HO – 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 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.


  • Kitmaster – Manufactured plastic model kits of railway engines, rolling stock, and buildings.
  • 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 (q.v.) bought the remainder. Most Airfix military vehicles are also to 1:76 scale.
  • Bachmann Branchline – One of the largest manufactures of ready to run 00.
  • Dapol – Produce kits and ready to run wagons.
  • Heljan – Produce a small number of locomotives and wagons.
  • Hornby Railways – One of the largest manufacturers of ready to run 00.
  • Lima – Produced budget 00 ready to run, bought by Hornby.
  • Peco – Produce a wide range of track.
  • Willets Scale Models, Produce hand-made, brass, carriages and wagons. (Link refers to a clothing supplier, now.)

External linksEdit