Terminology
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.
Read more about this topic: Rail Transport Modelling Scales