What is OO gauge?

In the early 20th century, many models for the British market were built to O gauge standards of 1:43 scale or 7mm:1ft. Though popular at the time, houses of the 1920s and 1930s were becoming smaller, the average living room measuring 16.01 sq.m. with an average of 3.21 bedrooms. Model railways remained within the preserve of the wealthy, but with smaller houses becoming more popular, a change of scale was required that would make building a model railway an achievable ambition in the average British house.

Launched in 1921 as 'The Table Railway', the first OO (Double-O) scale model railway system was to revolutionise the affordability and accessibility of railway models. A year later, in 1922, the first models of British prototypes appeared. There was a problem, however, because the clockwork mechanisms wouldn't fit inside the models. Many railways in Europe and North America run on the same 4ft 8 1/2in gauge track as established in Britain by the early railway pioneer George Stephenson, though the distance around the tracks to such objects as bridges, walls and tunnel ceilings (known as the loading gauge) was far more generous. This allowed continental locomotives to be wider and taller than their British counterparts.

If British models were to run on the same 16.5mm gauge track as their continental counterparts, the bodyshells would be smaller and the larger motors and gear mechanisms of the time would struggle to fit inside. Rather than make the model track wider and increase the scale of the entire model, the solution was found to be a compromise. By keeping the track gauge the same and increasing the size of the bodyshells from 1:87 (the common European scale) to 1:76, the motors and gear mechanisms of the era would fit without being on show.

Despite the advances in motor technology, the over-scale bodyshells have remained in the UK, a standard that modellers have come to expect and commonly refer to as OO gauge. 4mm:1ft scale also refers to OO gauge modelling, where a foot measurement in real life depicts 4mm in model form, as does 1:76, the ratio at which a real model should be reduced in size to depict an object in OO gauge. Track in OO gauge is set at a distance of 16.5mm between rails. Some modellers have sought to widen the gauge of the tracks to provide a more prototypical appearance, EM gauge being the most common with a track gauge of 18.2mm. P4 gauge at 18.83mm is to dead scale.

'Deadman's Lane' - a layout set in the 2010s, built to OO gauge standards.