In the world of model railways/railroads, the terms "scale" and “gauge” are frequently used interchangeably. Modellers often engage in conversations about models, modelling and model trains of various scales & sizes. Model railways are available in such a wide range of gauges & scales it can often be quite confusing for the newcomer. Model railway scales & gauges, range from the minuscule T scale to larger scales like O and G scales, etc. Amongst these, here in the United Kingdom, the OO scale or OO gauge stands as one of the more popular choices to model railways.
So what scale is British OO gauge?
British OO gauge or OO scale as it's often referred to is a modelling standard that represents a scale of 1:76 or, in millimetres, 4mm to the foot. It is seventy-six times smaller than the actual size of the real-life item being represented. Items sold as OO / 1:76 scale will be clearly marked or labelled as OO or 1:76 on their boxes or packaging. So a OO gauge item is the same scale (or perfectly compatible) with items marked as 1:76 scale.
What size is OO scale track?
The trackwork gauge for the OO scale is 16.5 mm, which is also compatible with the slightly smaller HO 1:87 scale. Both OO and HO scale trains can run on the same track and the track “gauge” refers to the distance between the centres of each rail. The only difference is that OO gauge model trains are slightly larger than HO scale models height-wise. Additionally and to complicate matters a little further, OO & HO scale tracks are available in various rail heights.
The standard height of OO scale track is referred to as Code 100, which represents a rail height of 0.1 inches (or in traditional terms 100 thou). When scaled up against real-life full-sized rail dimensions, the rail height in real life would be nearly 30% higher than its equivalent. This can be attributed to the early days of the OO scale when models featured coarser wheelsets with deeper flanges.
Some more demanding modellers prefer fine scale OO scale which is know as Code 75 track. Code 75 track has a rail height of 0.075 inches or 75 thou, which gives it a more accurate appearance. In real life, Code 75 would be approximately 3% lower when scaled up against its real-life full-sized rail. OO scale models today, most have much more finely scaled wheelsets so they'll run on both OO Code 100 & OO Code 75 tracks. If your trains have wheel sets with deeper flanges (older locos & wagons), make sure you use OO code 100 track only as they'll have issues with the flanges riding over the rail chairs on Code 75 track.
When purchasing any additional OO scale track for your model railway, make sure to check that it is the same height (Code 100 or Code 75) as the track you are using. Otherwise you'll here a huge clonk every time your trains run from code 100 to code 75 and vice versa! It'll also ruin them in the long run.