What is P4?

P4, S4, 18.83 - What do they mean?

P4 is short for Proto-four, OK so what's that? Well it is the Prototype standard gauge of four feet eight and a half inches, scaled down to 4mm to the foot or 1:76.2. It uses near scale wheel and flangeway dimensions, with minor differances to make it more practical for modellers.

S4 is short for Scale-four. It is similar to P4, but uses exact scale wheel and flangeway dimensions.
The differances between S4 and P4 are very minimal, down to about 0.01mm in most cases, and in my mind this is likely to be the working tolerance in either case, so the 2 are virtually identical, and interchangable, and as such I will always refer to P4 on this site.

You may have already guessed it, 18.83 is the track gauge of P4 measured in millimetres.

A little history, and why change to P4?

Firstly lets look at 00 gauge, this is the common gauge that ready to run British outline models use. 00 is a bit of a mess, many years ago a scale half the size of 0 (H0) was developed to a scale of 3.5mm to the foot, this was fine for continental models, but when modellers tried to build British models to this scale it was dicovered that the large motors of the time wouldn't fit inside British models due to their smaller size. As a result the bodies of British models were increased to 4mm to the foot scale, so that the motors would fit inside the models, unfortunately the H0 gauge was retained, this resulted in the gauge being to narrow for the models (a scale 7 inches). This resulted in many compromises having to built in to the models so that they would run on the narrow gauge, such as overwidth wheels, narrow frames, and others, but still the models don't look right due to the gross inaccuracy of the gauge.

Later on some modellers tried a slightly wider gauge of 18mm (EM) using the original 00 wheels, this was a slight improvement over 00, but still has a narrow gauge, over width wheels, narrow frames, and so on. EM is still in use today by modellers who think it is easier than P4 (see the P4 myths section), but now uses a gauge of 18.2mm due to problems with the original gauge.

00 and EM have both needed developing and improving throughout their history to fix errors in their original forms, and to ensure that trains will run properly without too many problems. This development continues today, as they still haven't got it right.

P4 was developed independently of model standards, and so does not contain the errors and compromises associated with the other gauges. P4 was developed by scaling down fullsize track and wheel dimensions. The standards have not needed improving or redrafting because they are a direct reduction of the proven standards of fullsize railways which have been in use for more than 100 years. By directly scaling down from the prototype we have a set of track and wheel standards that work and have a good relationship to each other, just as they do on fullsize railways, it also looks right, because it is!

P4 is the only standard for 4mm to the foot scale models, which uses the correct scale gauge, and has worked well since its inception, with no editing or redrafting having been done to the standards, because they are a direct reduction of the proven fullsize dimensions, and work perfectly.

So why model to P4 standards?

You model scenery at 4mm to the foot,

you model buildings at 4mm to the foot,

you model signals at 4mm to the foot,

you model locomotive, wagon, and carriage bodies at 4mm to the foot,

so your most of the way there already!

P4 track and wheel standards

Dimension Min Max TG Track Gauge 18.83* see GW GW Gauge Widening N/A 0.22 CG Check Gauge 18.15* 18.20 BC Between Checks 0.65 0.68* EF Effective Flange Thickness 0.35 0.40 BB Wheel Back-to-Back 17.67 17.75 TW Tyre Width 1.85 2.00

Compare the diagram on the left, to the table on the right to see what each of the dimensions refer to, all dimensions are in millimetres.

Dimensions TG, CG, CF, BC, and BB are controlled by construction gauges. Exactoscale also do a set of gauges which cover dimension GW.

Dimensions marked * are the recomended dimensions, which construction gauges are built to.

Dimension BB is recomended to be 17.7mm nominal.

For those who may not be aware, gauge widening is used on the prototype to enable long wheelbase loco's to negotiate tight curves, although I haven't found it necessary on models.