EM gauge

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EM
Scale per foot:	4mm to 1ft
Scale ratio:	1:76
Gauge:	18.2mm
Prototype Gauge:	Standard gauge

EM gauge is variant of 4 mm to a foot (1:76) scale used in model railways.

EM was developed because OO gauge, favoured by manufacturers of British prototype models, utilised track that 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 when scaled at the globally popular HO scale's 3.5mm to a foot (1:87). As the scale was increased to 4mm to the foot to make the locomotives larger, the track gauge was left at 16.5mm, and hence is too narrow (by a scale 178 mm) to correctly depict the prototype's track gauge of 4 foot 8½ inches.

EM gauge was founded in the 1950s, originally with 18mm gauge track and rolling stock wheelsets based upon the crude and massively out-of-scale products of the contemporary OO model manufacturers.

Since then wheelset standards have become more fine, allowing EM to evolve into 18.2mm gauge track (for a while called EEM gauge until it was adopted into the mainstream standard). Some modellers were still not happy with this and developed P4 scale standard (18.84mm gauge). In order to work the motion of outside-cylindered steam engines needs to be slightly overscale; in EM the track is slightly underscale and the flanks of the locomotive can be to scale; in P4 the track is to scale but flanks of the locomotives have to be overscale.

Most EM modellers will have started off using OO gauge and having acquired the necessary modelling skills, then advanced into EM. Modellers in EM typically re-wheel their rolling stock and hand-build their trackwork, although pre-built track is available from specialist suppliers. There are also many 4mm scale kits which can be used by all 4mm scale gauges, and since the advanced skills, advanced kitbuilding and scratchbuilding are also common.

EM standards are set by the EM Gauge Society, defining gauge and wheel dimensions to ensure compatibility across layouts.