Rail transport modelling scales

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. HO, OO, N, O, G, TT 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.

Although scale and gauge are often confused, scale means the ratio between a unit of measurement on a model compared with a unit of measurement in corresponding full size prototype, while 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 less than standard and broad-gauge railways where the gauge is wider. In a similar manner, 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.

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.

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.

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 are generally less reliable and more expensive to manufacture, which makes them unsuitable for mass-production products.

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.

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.

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).

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.

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.

The main railways in Great Britain use the international standard gauge of4 ft 8+1⁄2 in (1,435 mm) 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.

When H0 scale was being introduced, the motors available were too large to fit in scale-sized bodies and so as a compromise the scale was increased from 3.5 mm to 4 mm to the foot, but the gauge was not changed so other elements could be shared. For 00 therefore the track is about 12.5% narrower than it should be for the scale used. EM and P4 standards correct this anomaly by adopting a wider track gauge.

The globally more-widespread international NEM and NMRA scale standards are relatively rare in Great Britain and used almost exclusively by those modelling foreign prototypes.

While there are Japanese model railway manufacturers that export their products to other parts of the world and follow the scale standards of the export destinations, in Japan there are several domestic scales that are popular in the country but virtually unknown elsewhere. International NEM and NMRA scales are also used by some Japanese modellers. The main reason for the domestic scales different from international standards is the smaller prototype loading gauge and unusual gauges of Japanese railways: 600 mm (1 ft 11+5⁄8 in), 762 mm (2 ft 6 in) and 1,067 mm (3 ft 6 in) are used, along with standard gauge of 1,435 mm (4 ft 8+1⁄2 in).

Lego trains use a fixed nominal gauge of 37.5 mm (1+15⁄32 in), based on 5-stud track centerlines gauge.

The 37.5 mm length is not derived by a certain scale ratio. While HO scale is a 1:87 scale (3.5 mm to 1 foot), resulting in a 16.5 mm (0.65 in) gauge from real life prototype 1,435 mm (4 ft 8+1⁄2 in) standard gauge standard gauge.

Conversely, modeling standard gauge in Lego trains would yield a scaling of (37.5:1435 =) 1:38.3.

Live steam model railways are not standardized systematically by any single standardization body. There are, however, certain scales and gauges which have become de facto standards and in some cases correspond to either NEM or NMRA standard scales. One example is the "IBLS" (International Brotherhood of Live Steamers), an informal organization which has published standards for some of the gauges. Many clubs have their own standards, which also may vary slightly from country to country. Hornby Railways have pioneered commercial model live steam in 00 (1:76 scale on 16.5 mm gauge), the existing models are heated using a controllable electric current through the two running rails and have the steam pressure chamber in the model tender.

In addition to these scales, the United Kingdom has, over the last forty years, fathered a scale that is based on the predominant British narrow track gauge of 2 ft (610 mm). Using 32 mm (1.26 in) - 0 gauge - track, there is an extensive range of 16 mm to the foot scale [1:19] live-steam and other types of locomotives, rolling stock and accessories. Many of these models are dual gauge, and can be converted to run on 45 mm (1.772 in) track (gauge 1), and radio control is common. Locomotives in this scale are generally large and "chunky", and can range from the tiny 0-4-0 seen on Welsh slate quarry lines all the way up to the very largest found in the UK, such as the ex-ACR NG/G16 Beyer-Garratt locomotives, seen running on the Welsh Highland Railway in North Wales. The hobby is supported by a number of 16 mm live steam and electric traction builders, dominated by the likes of Roundhouse Engineering and Accucraft UK.

There have been many short-lived and often promising model railway scales which are very much defunct nowadays. Quite often these were backed by only the company that created a new scale in the first place.

It is possible to use different scales of models together effectively, especially to create a false sense of depth (referred to as "forced perspective"). Scales close to each other are also hard to tell apart with the naked eye. An onlooker seeing a 1:43 model car next to a 1:48 scale model train would probably not notice the difference.

Some common examples of mixing scales are:

• a foreshortening technique using N scale (1:160) model trains in the background (distance) with HO scale (1:87) in the foreground.
• mixing 1:43 scale, 1:48 scale and 1:50 scale die-cast models with O scale model trains.
• using Matchbox cars (1:64 to 1:100) with HO scale and S scale.
• mixing OO scale British model trains with HO scale models. Both scales run on the same track but OO is slightly larger in scale.
• using 1:144 scale die-cast models with N scale.

• Rail transport modelling
• List of scale model sizes
• The Museum of the Moscow Railway
• National Model Railroad Association
• Normen Europäischer Modelleisenbahnen
• Rail transport modelling
• List of narrow gauge model railway scales

• http://www.spikesys.com/Modelrr/scales.html – Contains more specifics about some of the scales
• https://dfarq.homeip.net/sizes-of-model-trains/ – Contains photos of human hand comparison to each scale
• http://modeltrains.about.com Archived 2017-01-29 at the Wayback Machine – Online resource for model railroaders
• The Gauge One Model Railway Association
• The Gauge 3 Society
• Mundo Ferroviario Archived 2009-05-01 at the Wayback Machine, Web portal dedicated to a modelling in HO scale (Spanish).
• "About Gauge" guide from Lionel discussing O versus O27 gauge
• "More About Gauge" guide from Lionel discussing gauges other than O