4mm scale

4 mm scale is the most popular model railway scale used in the United Kingdom. The term refers to the use of 4 millimeters on the model equating to a distance of 1 foot (305 mm) on the prototype (1:76.2). It is also used for military modelling.

For historical reasons, a number of different standards are employed.

tandard gauge

Three different gauges are used for modelling standard gauge railways in the United Kingdom.

OO gauge

OO gauge uses 4 mm scale with 16.5 mm gauge track, which is inaccurately narrow since it is correct for HO scale (1:87.1). It is the most popular standard for 4 mm scale trains and is the only one produced by mass-market manufacturers. The traditional standard for wheels and track is a very coarse one with extremely oversize rails and flanges; in recent years, some manufacturers have switched to using the American National Model Railroad Association HO standard S-4 instead.

EM gauge

EM was originally was defined to use 18mm as the track gauge (hence the name), this was revised, and today EM gauge uses an 18.2mm track gauge, which is closer to accurate but not fully to scale. It was developed as an early improvement on the standard OO system. It is still popular with FineScale modellers today because it utilises larger than scale flanges on the wheels of rolling stock, and because point and crossing (P&C) work is a little easier due to an overscale flangeway clearance. These two concessions in scaling down the prototype make for reliable running. This factor explains the continued popularity of EM as a scale for finescale modellers working today.

P4 gauge

P4 is a close to exact 4 mm:1 ft (305 mm) replica of real-life track and wheel dimensions. The gauge of track built to these standards is 18.83mm. Along with EM Gauge It is also a popular choice for finescale modellers working today. The main advantage of P4 over the other 4mm standards is that the wheel flanges are approximately to scale and the flangway gaps on P&C work are also close to scale. This is most noticeable in photographs of the model where in many cases is it almost impossible to detect that you are looking at a model, as opposed to the real thing. The disadvantages of near to scale flanges and flangeway crossings are that the trackwork has to be laid and built to very close tolerances and needs to be flat. If this is not the case then the reliability of rolling stock on the trackwork is compromised leading to poor running and derailments. This issue can, to an extent, be mitigated by the used of a suspension system in the locomotives and by the use of 3 point suspension system in the rolling stock used on the layout.

Conversion to EM and P4

00 models can be converted to work on EM or P4 by replacing or reprofiling wheels, fitting the correct width axles and moving the locomotive or carriage frames outward to suit. Modern products from Hornby, Bachmann UK and others have some element of springing in their suspension to aid this, and many aftermarket chassis kits exist from a variety of vendors such as high level kits, Comet, DJH, and others. Products intended for the more dedicated modeller are generally designed to be buildable to fit any of the three standards, though it should be noted that one side effect of underscale locomotive frames for 00 gauge is that the width between the frames is limited, which can limit the size of electric motor that may be fitted.

Irish Broad Gauge

Irish Broad Gauge is 5' 3". OO models are available from Bachmann and have previously been available from Lima. P4 standards can also used with a track gauge of 21mm.

Narrow gauge

OO9

9 mm gauge (N gauge) track used to model 2' - 2' 6" gauge prototypes.

OOn3

12 mm gauge (TT gauge) track used to mode 3 foot (914 mm) prototypes which were common in Ireland and the Isle of Man.

External links

* [http://www.willetsscalemodels.co.uk/ Willets Scale Models] , Specialist manufacturer in 4mm scale railway stock.
* [http://www.doubleogauge.com/ The Double 0 Gauge Association] , supports 4mm modelling using a 16.5mm gauge
* [http://www.emgs.org/ EM Gauge Society] , supports modelling in both EM (18.2 gauge) and P4 (18.83 gauge)
* [http://www.scalefour.org/ The Scalefour Society] , supporting modelling in P4
* [http://www.demu.co.uk/ Diesel and Electric Modellers United] , supporting post steam era modelling in the UK
* [http://www.009society.com/ The 009 Society] - Supporting modelling 4mm narrow gauge modelling