EM gauge
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EM gauge is a gauge of model railway, 4 mm scale. It was developed because OO gauge was too narrow. OO was developed in the UK in the 1930s as a response to manufacturers finding they were unable to fit the motors of the time into British prototype small boilered locomotives scaled at 3.5 mm to the foot (known as HO scale). As the scale was increased to 4 mm to the foot to make the locomotives larger, the gauge was left at 16.5 mm, and hence is too narrow (by a scale 178 mm) to correctly depict the prototype's track gauge of 4 foot 8½ inches.
EM was has evolved since the 1950s as a gauge closer to the prototypically correct 18.83 mm (modellers following Scalefour (S4) or Protofour (P4)standards use this track gauge). EM or "Eighteen Millimetres" actually has a gauge of 18.2 mm, and is modelled at 4 mm to the foot. Modellers in EM typically rewheel their rolling stock and hand build their trackwork. A set of standards is available defining gauge and wheel dimensions to ensure compatibility across layouts.
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