P4 gauge

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It has been suggested that Irish P4 be merged into this article or section. (Discuss)

P4
Scale per foot:	4mm to 1ft
Scale ratio:	1:76
Gauge:	18.83mm
Prototype Gauge:	Standard gauge

P4 gauge or Protofour gauge is a set of standards for model railways allowing the 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 manufacturers of British prototype models use the inaccurate OO gauge (1:76 models running on 16.5mm gauge track). P4 represents the end point of a movement that started in the 1950s to model the 4mm to a foot scale accurately.

For standard gauge track the P4 standards specify a scale model track gauge of 18.83 mm. As well as a track gauge, P4 also specifies the exact wheel profile and track parameters to use, which are largely an exactly to scale version of real life standards.

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.

Irish P4, consists of 5 ft 3 in (1,600 mm) Irish broad gauge modelled in 4mm scale with 21mm gauge track. Several successful models of narrow gauge prototypes with a correspondingly accurate track gauge 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.

Owing to space constraints and the necessity for the construction in model form of tighter than prototypical track curves, there are still some compromises within P4 standards. For some railway modellers who feel that this is unnecessary an exact set of standards has been formulated and are known as Scalefour or S4 standards. Owing to the operational difficulties caused by such strict adherence to scale, S4 is not popular compared to P4. Rather confusingly, the Scalefour Society mostly promotes P4 standards.