Editing Rail transport modelling (section)

== Scales and gauges ==
{{Main|Rail transport modelling scales|Rail transport modelling standards}}
{{comparison_of_model_railway_scales.svg}}

The words ''scale'' and ''gauge'' seem at first interchangeable but their meanings are different. ''Scale'' is the model's measurement as a proportion to the original, while ''gauge'' is the measurement between the rails.

The size of engines depends on the scale and can vary from {{convert|700|mm|in|1|abbr=on}} tall for the largest rideable [[live steam]] scales such as 1:4, down to matchbox size for the smallest: [[Z scale|Z-scale]] (1:220) or [[T scale]] (1:450). A typical [[HO scale|HO (1:87)]] engine is {{convert|50|mm|in|2|abbr=on}} tall, and {{convert|100|to|300|mm|in|2|abbr=on}} long. The most popular scales are: [[G scale]], [[Gauge 1]], [[O scale]], [[S scale]], [[HO scale]] (in Britain, the similar [[OO gauge|OO]]), [[TT scale]], and [[N scale]] (1:160 in the United States, but 1:148 in the UK). HO and OO are the most popular. Popular narrow-gauge scales include [[Sn3]], [[HOn3]] and [[Nn3]], which are the same in scale as S, HO and N except with a narrower spacing between the tracks (in these examples, a scale {{RailGauge|3ft}} instead of the {{RailGauge|4ft8.5in}} standard gauge).

The largest common scale is 1:8, with 1:4 sometimes used for park rides. [[G scale]] (Garden, [[1:24 scale]]) is most popular for backyard modelling. It is easier to fit a G scale model into a garden and keep scenery proportional to the trains. [[Gauge 1]] and Gauge 3 are also popular for gardens. O, S, HO, and N scale are more often used indoors.<ref>{{Cite web|url=http://www.oakridgehobbies.com/plastic-wood-hobby-model-kits/modeling-in-scale-dimensions-f-a-q-s.html|title=MODELING IN SCALE – Dimensions, Conversion Charts, Sizes, F.A.Q.s|website=www.oakridgehobbies.com|access-date=2016-11-17|archive-date=2016-11-17|archive-url=https://web.archive.org/web/20161117210940/http://www.oakridgehobbies.com/plastic-wood-hobby-model-kits/modeling-in-scale-dimensions-f-a-q-s.html|url-status=dead}}</ref><ref>{{Cite web|url=http://www.sawyoo.com/post_model-train-scales-chart_338195/|title=Best Photos of Model Train Scales Chart - Scale Model Conversion Chart, Model Railroad Scales Comparison and Scale Model Trains / sawyoo.com|website=www.sawyoo.com|access-date=2016-11-17}}</ref>
{| class="wikitable"
!Scale
!Ratio
|-
|T
|1:450
|-
|ZZ
|1:300
|-
|Z
|1:220
|-
|N
|1:160
|-
|2mm
|1:152
|-
|TT
|1:120
|-
|3mm
|1:101
|-
|HO
|1:87

|-
|OO
|1:76.2
|-
|S
|1:64
|-
|O
|1:48
|-
|1
|1:32
|-
|G
|1:22.5
|}
At first, model railways were not to scale. Aided by trade associations such as the [[National Model Railroad Association]] (NMRA) and ''[[Normen Europäischer Modellbahnen]]'' (NEM), manufacturers and hobbyists soon arrived at ''[[de facto]]'' [[standardisation|standards]] for interchangeability, such as gauge, but trains were only a rough approximation to the real thing. Official scales for the gauges were drawn up but not at first rigidly followed and not necessarily correctly proportioned for the gauge chosen. 0 (zero) gauge trains, for instance, operate on track too widely spaced in the United States as the scale is accepted as 1:48 whereas in Britain 0 gauge uses a ratio of 43.5:1 or 7&nbsp;mm/1 foot and the gauge is near to correct. British OO standards operate on track significantly too narrow. The 4&nbsp;mm/1 foot scale on a {{RailGauge|16.5mm}} gauge corresponds to a track gauge of {{RailGauge|4ft1.5in}}, {{convert|7|in|mm|0|disp=or}} (undersized). {{RailGauge|16.5mm}} gauge corresponds to {{RailGauge|4ft8.5in}} standard gauge in H0 (half-0) 3.5&nbsp;mm/1 foot or 1:87.1. This arose due to British locomotives and rolling stock being smaller than those found elsewhere, leading to an increase in scale to enable H0 scale mechanisms to be used. Most commercial scales have standards that include wheel [[flange]]s that are too deep, wheel treads that are too wide, and [[rail tracks]] that are too large. In H0 scale, the rail heights are codes 100, 87, 83, 70, 55, 53, and 40 -- the height in thousandths of an inch from base to railhead (so code 100 is a tenth of an inch and represents 156-pound rail).

Later, modellers became dissatisfied with inaccuracies and developed standards in which everything is correctly scaled. These are used by modellers but have not spread to mass-production because the inaccuracies and overscale properties of the commercial scales ensure reliable operation and allow for shortcuts necessary for cost control. The finescale standards include the UK's P4, and the even finer S4, which uses track [[dimensions]] scaled from the prototype. This 4&nbsp;mm:1&nbsp;ft modelling uses wheels {{convert|2|mm|in|3|abbr=on}} or less wide running on track with a gauge of {{RailGauge|18.83mm}}. [[Railroad switch#Guard rail .28check rail.29|Check-rail]] and wing-rail clearances are similarly accurate. 

A compromise of P4 and OO is "EM" which uses a gauge of {{RailGauge|18.2mm}} with more generous tolerances than P4 for check clearances. It gives a better appearance than OO though pointwork is not as close to reality as P4. It suits many where time and improved appearance are important. There is a small following of finescale OO which uses the same 16.5mm gauge as OO, but with the finer scale wheels and smaller clearances as used with EM- it is essentially 'EM-minus-1.7mm.'