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 a letter 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 NEM or 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.

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