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

 

Collectable Models

These were generally of a similar size to scale models. However, the scale could vary considerably from the early Trix models to the more accurate Hornby Dublo  Wrenn and Triang models. As time passed many of these brands became more accurate and would now fall into the scale category. Initially, a lot of the passenger carriages, in particular, were foreshortened to enable them to run around the fairly tight radius train set curves both Hornby Dublo and Trix Twin use the three rail contacts system that this gradually became obsolete. For collectors today, however, the nostalgia of this system remains.