View source for 4 mm scale

{{Short description|Model railway scale}}
{{more citations needed|article|date=April 2022}}
{{Infobox model rail scale
| name = 4 mm scale
| image_filename =OO Scale BR 25054.JPG
| image_caption =4 mm scale model of a [[British Rail Class 25]] shown with a [[British five pence coin|18mm coin]] for size.
| scale = 4&nbsp;mm to {{cvt|1|ft|mm|0}}
| standard =
| ratio =1:76.2
| gauge =
| prototype_gauge =
}}'''4&nbsp;mm scale''' is the most popular<ref>{{Cite web |title=OO - World Of Railways |url=https://www.world-of-railways.co.uk/Model-Railways/OO |access-date=2022-03-31 |website=www.world-of-railways.co.uk}}</ref> [[Rail transport modelling scales|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&nbsp;mm) on the [[Prototype#Scale modeling|prototype]] (1:76.2). It is also used for [[Model military vehicle|military modelling]].

For historical reasons, a number of different standards are employed.<ref>{{Cite web |date=2019-01-16 |title=Model Railway Scales and gauges explained |url=https://www.world-of-railways.co.uk/techniques/model-railway-scales-and-gauges-explained/ |access-date=2022-03-31 |website=World Of Railways}}</ref>

==Standard gauge==
Three different gauges are used for modelling [[standard gauge]] railways in the [[United Kingdom]].

===OO gauge===
[[OO gauge]] uses 4&nbsp;mm scale with 16.5&nbsp;mm [[Rail gauge|gauge]] track,<ref>{{Cite web |title=Standards |url=https://doubleogauge.com/standards/ |access-date=2022-03-31 |website=The Double O Gauge Association |language=en-GB}}</ref><ref>{{Cite web |date=2019-06-07 |title=What is OO gauge? |url=https://www.world-of-railways.co.uk/techniques/what-is-oo-gauge/ |access-date=2022-03-31 |website=World Of Railways}}</ref> which is inaccurately narrow since it is correct for [[HO scale]] (1:87.1). It is the most popular standard in the UK for 4&nbsp;mm scale trains<ref>{{cite web |title=OO gauge |url=https://www.fabtintoys.com/oo-gauge/ |website=Fabtintoys |publisher=Fabintoys |access-date=12 April 2022}}</ref> and is produced by the two main manufacturers in the UK. The traditional standard for wheels and track is a very coarse one with extremely oversize [[Rail tracks|rail]]s and [[flange]]s; in recent years, some manufacturers have switched to using the American [[National Model Railroad Association]] HO standard S-4 instead.

===EM gauge===
[[EM gauge|EM]] was originally defined to use 18mm as the track gauge (hence the name: Eighteen Millimetres). This was revised, and today EM gauge uses an {{RailGauge|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===
[[Protofour|P4]], otherwise known as [[Protofour]], or "18.83", is a close to exact 4&nbsp;mm:1&nbsp;ft replica of real-life track and wheel dimensions.<ref>{{Cite web |last=Elliott |first=Russ |title=Scalefour Digest 1.2 'P4 Track and Wheel Standards' |url=http://www.clag.org.uk/p4standards.html |access-date=2022-03-31 |website=www.clag.org.uk |language=en-GB}}</ref> The gauge of track built to these standards is 18.83&nbsp;mm.  Along with EM Gauge It is also a popular choice for finescale modellers working today.  The main advantage of P4 over the other 4&nbsp;mm standards is that the wheel flanges are approximately to scale and the flangeway 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 problem can, to an extent, be mitigated by the use of some form of suspension system in the locomotives and rolling stock used on the layout. There are different ways to do this and reference to one of the societies supporting modelling to this standard is recommended.{{According to whom|date=February 2024}}

==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 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.{{cn|date=February 2024}}

==Irish Broad Gauge==
[[Rail gauge in Ireland|Irish Broad Gauge]] is 5' 3". [[OO gauge|OO]] models are available from [[Bachmann Branchline|Bachmann]] and have previously been available from [[Lima (models)|Lima]]. [[Protofour|P4 standards]] can also be used with a track gauge of 21&nbsp;mm.{{cn|date=February 2024}}

==Narrow gauge==
{{see also| Narrow gauge}}
[[Image:009 Gryndene Halt.jpg|thumb|right|200px|Gryndene Halt, a station on Evaleight Light Railway, by the Sussex Downs 009 group.]]

===OO9===
{{see also|OO9}}
9&nbsp;mm gauge ([[N gauge]]) track used to model 2'–2'6" gauge prototypes.

===OOn3===
{{see also|OOn3}}
12&nbsp;mm gauge ([[TT gauge]]) track used to model {{convert|3|foot|mm}} prototypes which were common in [[Ireland]] and the [[Isle of Man]].

==References==
{{reflist}}

==External links==
* [http://www.doubleogauge.com/ The Double 0 Gauge Association], supports 4&nbsp;mm modelling using a 16.5&nbsp;mm 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 (18.83 gauge)
* [http://www.demu.org.uk/ Diesel and Electric Modellers United], promoting and supporting the modelling of the Diesel and Electric era of the British standard gauge railway system
* [http://www.009society.com/ The 009 Society] - Supporting modelling 4&nbsp;mm narrow gauge modelling

{{Scale models}}

[[Category:4 mm scale| ]]
[[Category:Scale model scales]]