P4 gauge

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P4
Scale 4 mm to 1 ft
Scale ratio 1:76.2
Model gauge 18.83 mm (0.741 in)
Prototype gauge Standard gauge

P4 or Protofour is a set of standards (of which the article name "P4 gauge" is a misnomer as the standards concern much more than simply the gauge of the track) for model railways allowing a more accurate construction of models to a scale of 4 mm to 1 foot (1:76.2), the predominant scale of model railways of the British prototype. For historical reasons almost all manufacturers of British prototype models use the inaccurate 00 gauge (1:76.2 models running on under-scale 16.5 mm (0.65 in) gauge track). P4 represents the most significant development of a movement that started in the 1950s to model accurately to the 4 mm to 1 foot scale.

The P4 standards specify a scale model track gauge of 18.83 mm (0.741 in) for standard gauge railways. The late Joe Brook Smith was the first to use of an exact scale track gauge in July 1964, when also the term “Protofour” was invented by Malcolm Cross. The standards were later published in Model Railway News by the Model Railway Study Group in August 1966.

Just as in the prototype railway, on a model the wheel-rail interface is the fundamental aspect of reliable operation. So as well as a track gauge, P4 also specifies the wheel profile and track parameters to use, which are largely a scaled down version of real life standards with some allowances for practical manufacturing tolerances.

P4 standards have been extended to several other prototypes. Broader than standard gauges have been modelled using P4 standards, including Brunel's 7 ft 1⁄4 in (2,140 mm) gauge, modelled with 28.08 mm (1.106 in) track and Irish P4, the 5 ft 3 in (1,600 mm) Irish broad gauge modelled in P4 in 4 mm scale with 21 mm (0.827 in) gauge track. Several successful models of narrow gauge prototypes with a correspondingly accurate track gauges have also been produced to P4 standards.

P4 standards are promoted worldwide by the Scalefour Society, which is based in the United Kingdom. The EM Gauge Society also provides support for modelling to P4 standards: many P4 modellers belong to both societies. The standards can be downloaded from download P4-standards and also viewed at P4 standards

The P4 standards include options with reduced tolerances and an increased back-to-back measurement that are an exact scaling down of prototype dimensions, and which are referred to as S4 standards. These are chosen by railway modellers who wish to avoid the minor compromises designed into P4 standards.

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