OO 4mm 

00 or 4 mm scale

is the most common in British models. The history behind this is complex but is related to the size of British loading gauge which was significantly smaller than continental European and American prototypes. As a result of this size difference, the British toymakers enlarged the models to fit the early electric motors into the body of the locomotives. They maintained the 16.5 mm track gauge or 5/8 of an inch for compatibility with other models. This compromise continues to this day, although for the true scale modeller EM gauge at 18.83 mm has become the standard for those who do not wish to compromise on anything. However, 00 continues to be the predominant and most popular method of modelling British prototype

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