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 4 ft 8+1⁄2 in (1,435 mm) 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 (6.5 mm / 0.256 in), whereas the same standard gives only 5% for 45 mm (1.772 in) 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 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. NEM-standards are used by model railway industry and hobbyists in Europe. The standards are published in French and German and both versions have an official status. Unofficial translations in English from third parties exist for certain NEM-standard sheets. 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 (1,250–1,700 mm or 49.2–66.9 in) m = metre gauge (850–1,250 mm or 33.5–49.2 in) e = narrow gauge (650–850 mm or 25.6–33.5 in) i = industrial (400–650 mm or 15.7–25.6 in) p = park railway (300–400 mm or 11.8–15.7 in) For instance, a metre-gauge model railway in H0-scale is designated H0m. In German text the letter "f" (for Feldbahn) is sometimes used instead of "i". The letter "e" represents the French word for "narrow", étroit. 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).
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