Rail transport modelling uses a variety of scales (ratio between the real world and the model) to ensure scale models look correct when placed next to each other. Model railway scales are standardized worldwide by many organizations and hobbyist groups. Some of the scales are recognized globally, while others are less widespread and, in many cases, virtually unknown outside their circle of origin. Scales may be expressed as a numeric ratio (e.g. 1/87 or 1:87) or as letters defined in rail transport modelling standards (e.g. G, O, S, OO, HO, TT, N, and Z.) The majority of commercial model railway equipment manufacturers base their offerings on Normen Europäischer Modellbahnen (NEM) or National Model Railroad Association (NMRA) standards in most popular scales.
Terminology Scale and gauge are different: scale means the ratio between a unit of measurement on a model compared with a unit of measurement in corresponding full size prototype, whereas gauge is the distance between the two running rails of the track. About 60% of the world's railways have a track gauge of known as "standard gauge", but there are also narrow-gauge railways where the track gauge is narrower than standard, and broad-gauge railways where the gauge is wider. Similarly, a scale model railway may have several track gauges in one scale.
In addition to the scale and gauge issue, rail transport modelling standards are also applied to other attributes such as catenary, rolling stock wheel profile, loading gauge, curve radii and grades for slopes, to ensure interoperation of scale models produced by different manufacturers. Globally, the two dominating standard organizations are NMRA in North America and MOROP in Europe with their NEM standard.
History of scale standards
The first model railways were not built to any particular scale and were more like toys than miniature representations. Eventually, models became more accurate, and benefits of standardization became more obvious. The most significant and the most basic area of standardization was the model track gauge. At first, certain gauges became de facto standards for hobbyists and manufacturers. While the first unofficial standard gauges made interchangeability possible, the models were still only a rough approximation of rolling stock.
Eventually unofficial or manufacturer-specific scale standards became more established, and model railway standardisation bodies such as the NMRA and MOROP formalised them. However, they were very often poorly implemented in design and manufacturing processes with commercial manufacturers before the World War II. The conformity to scale standards grew strongly in the 1950s and 1960s when many new model railway accessories manufacturers were born and to whom the standard conformity was vital.
Inaccuracy to improve reliability For most standardized model railway scales, the nominal scale reduction ratio is not applied systematically to all the components of a scale model railway, and normally the standards give scale specific design guidelines for all the scales they cover. Reliability of operations requires that certain parts be made oversize. A typical example is the wheel flanges, which must be proportionally higher in smaller scales to ensure that lighter and smaller models do not derail easily as they would if universal flange proportions were used in all the scales. For instance, a Z scale wheel flange as defined in the NEM standard should be about 9% of the scale nominal standard gauge (), whereas the same standard gives only 5% for standard-gauge I scale.
Finescale standards Modellers who were dissatisfied with inaccuracies in the name of reliability have developed alternative finescale standards. Finescale standards are very much restricted to discerning hobbyists since, by definition, finescale model railways require finer tolerances and are therefore more expensive to manufacture, which makes them unattractive for mass-production product manufacturers.
Linear propulsion One limitation with smaller scales is that the small size of the metal contacts means it is easier for dust and dirty track to interfere with the electrical circuit needed to drive the train. Moreover, the tight tolerances also mean friction can more easily interfere with the train moving. One approach to enable further miniaturization beyond T Scale is the recent commercial introduction of linear drive motors. The magnetic propulsion eliminates moving parts, simplifying motion and models. Commercially available scales using linear propulsion drive range from 1:655 to 1:1000.
Scale standards Most standards are regional, but some have followers in other parts of the world outside their native region, most notably NEM and NMRA. While the most significant standardised dimension of a model railway scale is the gauge, a typical scale standard covers many more aspects of model railways and defines scale-specific dimensions for items like catenary, rolling stock wheels, loading gauge, curve radii and grades for slopes, for instance.
NEM MOROP (the European federation of national model railway associations) is a European organisation which publishes NEM-standards. The standards are used by model railway industry and hobbyists in Continental Europe. The NEM standards are published by the technical commission of MOROP in French and German and both versions have an official status. Member associations and individuals have translated these official versions into other languages, and therefore English versions are also available directly from MOROP. Due to the unofficial status, some translations are just literal (word-by-word) translations. For example, in NEM 010 the French voie H0 translates as gauge H0 in English, the same as the German Spur H0.
Model railway scales and gauges are standardized in NEM 010, which covers several gauges for each scale. Narrow gauges are indicated by an additional letter added after the base scale as follows: no letter = standard gauge () m = metre gauge () e = narrow gauge () i = industrial () p = park railway () For instance, a metre-gauge model railway in H0-scale is designated H0m. In German text the letter "f" (for ) is sometimes used instead of "i". The letter "e" represents the French word for "narrow", . NEM gauges are arranged conveniently to use the normal gauge of smaller scales as narrow gauges for a certain scale. For instance, H0m gauge is the same as the TT-scale normal gauge, H0e same as the N-scale normal gauge and H0i same as the Z-scale normal gauge.
For H0 and 0 scales, NEM uses the number zero, and NMRA uses letter "O" (HO instead of H0).
Scale Ratio Standard gauge m e i p Notes Z 1:220 4.5 mm – – – Standard gauge based on the Märklin factory standards. N 1:160 6.5 mm 4.5 mm – – Standard gauge based on the Arnold factory standards. TT 1:120 9 mm 6.5 mm 4.5 mm – – H0 1:87 12 mm 9 mm 6.5 mm 4.5 mm "Half zero" S 1:64 16.5 mm 12 mm 9 mm 6.5 mm – 0 1:45 22.5 mm 16.5 mm 12 mm 9 mm 1 1:32 32 mm 22.5 mm 16.5 mm 12 mm – II 1:22.5 45 mm 32 mm 22.5 mm 16.5 mm Standard gauge known as Gauge 3 in the UK (scaled at inch to the foot) and known as Gauge on the Märklin factory standards from 1891. III 1:16 63.5 mm 45 mm 32 mm 22.5 mm V 1:11 127 mm (5 in) 89 mm 63.5 mm 45 mm 32 mm Common gauge for Live Steam scaled at inches to the foot in the UK VII 1:8 127 mm 89 mm 63.5 mm 45 mm Common gauge for Live Steam X 1:5.5 184 mm 127 mm 89 mm 63.5 mm –
NMRA The NMRA (National Model Railroad Association) standardized the first model railway scales in the 1940s. NMRA standards are used widely in North America and by certain special interest groups all over the world. To some extent NMRA and NEM standards are compatible, but in many areas, the two standards specify certain model railway details in somewhat incompatible ways for the same scale.
There are two NMRA standard sheets where the scales have been defined. NMRA standard S-1.2 covers the popular model railway scales and S-1.3 defines scales with deep flanges for model railways with very sharp curves or other garden railway specific design features.
In certain NMRA scales an alternative designation is sometimes used corresponding the length of one prototype foot in scale either in millimetres or in inches. For instance, 3.5 mm scale is the same as HO. For HO and O -scales, NMRA uses the letter "O" whereas NEM uses the number zero (H0 instead of HO).
The NMRA published alternative, more accurate and realistic standards for track and wheels sheet in S-1.1. These model railway standards are based on the full size prototype standards and the scale model operational reliability is therefore reduced in comparison to the models conforming to the normal NMRA standards. Proto and finescale rails and wheels are generally not compatible with the normal scale model railway material with the same scale ratio.
Proto scale was originally developed by the Model Railway Study Group in Great Britain in 1966 and later adopted into NMRA standards with modifications necessary for the North American prototype railway standards. Proto scale reproduces faithfully the prototype wheel tread profile and track work used by the Association of American Railroads and the American Railway Engineering Association.
Finescale reproduces the prototype wheel tread profile and track work used by the Association of American Railroads and the American Railway Engineering Association with minor compromises for performance and manufacturability.
NMRA popular railway scales Scale Ratio Model gauge Notes Z 1:220 NMRA does not give any other dimensions for Z scale apart from the gaugeThe s.g. is set nominally to gauge=6.5 mm; more exact to 1:220 would be Nn2 1:160 narrow gauge Nn3 1:160 narrow gauge N 1:160 standard gauge TT 1:120 standard gauge HOn2 or 3.5 mm 1:87.1 narrow gauge HOn30 or 3.5 mm 1:87.1 (0.353 in) narrow gaugeN-scale track can be used HOn3 or 3.5 mm 1:87.1 narrow gauge HOm or 3.5 mm 1:87.1 narrow gaugeTT-scale track can be used HO or 3.5 mm 1:87.1 standard gauge OO or 4 mm 1:76.2 standard gauge Sn3 or 3/16 in 1:64 narrow gauge S or 3/16 in 1:64 standard gauge On2 or 1/4 in 1:48 narrow gauge On30 or 1/4 in 1:48 HO-track narrow gauge On3 or 1/4 in 1:48 (?) narrow gauge O or 1/4 in 1:48 is true standard gauge No. 1n2 or 3/8 in 1:32 narrow gauge (same as On3 gauge) No. 1n3 or 3/8 in 1:32 narrow gauge No. 1 or 3/8 in 1:32 standard gauge Fn3 or 15 mm 1:20.32 No. 1-track narrow gauge F or 15 mm 1:20.32 Identical to Proto 20.32 except the wheel flange depth 3/4 in 1:16 standard gauge 1 in 1:12 Common gauge for Live Steam Note: to interpret the number in the left-hand column, these examples illustrate: 3.5 mm scale (HO): 3.5 mm scale measurement = 1 foot (304.8 mm) prototype. The ratio is therefore 1:87.08571, usually reported as 1:87. 1 in scale: 1 in scale measurement = 1 foot prototype, the ratio is reported as 1:12.
NMRA deep flange scales Scale Ratio Gauge Notes SHR or 3/16 in 1:64 vs. – O27 – – Same as OHR but models 10% smalleron the same track gauge OHR or 1/4 in 1:48 – G or 3/8 in 1:32 – G 1:29 – G 1:24 – G 1:22.5 – G 1:20.3 –
NMRA proto scales Scale Ratio Gauge Notes Proto:20.32 1:20.32 NMRANMRA 70.64mmNMRA 70.69mm Proto:20.32n3 1:20.32 – Proto:32 1:32 – Proto:32n3 1:32 – Proto:48w5 1:48 Russian prototypes Proto:48 1:48 – Proto:48n3 1:48 – Proto:64 1:64 – Proto:64n3 1:64 – Proto:87 1:87.1 – Proto:87n3 1:87.1 –
NMRA finescale Scale Ratio Gauge Notes Fine:HO 1:87.1 – Fine:HOn3 1:87.1 – Fine:TT 1:120 – Fine:N 1:160 – Fine:Nn3 1:160 –
British The main railways in Great Britain use the international standard gauge of but the loading gauge is narrower and lower than in the rest of Europe with the same standard gauge. This is one of the main reasons why the country has traditionally used its own distinctive model railway scales which can rarely be found outside the British Isles.