I am afraid that some of the comments on 'scale' are incorrect. HO (1:87) scale is 3.5mm/ft. this is the most common scale that the rest of the world uses in railway modelling. OO is a mixture of scales. The model its self is (1:76) scale 4mm/ft, but the track gauge - how far the wheels are apart, is (1:87) scale 3.5mm/ft. This came about due to not being able to fit, the then available size of electic motors in the early days of railway modelling, into the models, because of the smaller loading gauge of British locomotives compared to the rest of the world. A compromise was made in the form of 'OO scale' by Frank Hornby I believe, that the U.K. has been stuck with ever since. OO scale should therefore correctly be described as, 4mm scale narrow gauge. You only have to look at any OO locomotive head on, compared to a similar HO locomotive, to see the difference. How any manufacturer can describe a model locomotive, vehicle etc. as being both OO and HO scale at the same time, is therefore one of the great mysteries of life.