Model railroading (US) or Railway modelling (UK, Australia and Canada) is a hobby in which rail transport systems are modelled at a reduced scale, or ratio. The scale models include rail vehicles (locomotives, rolling stock, streetcars, etc.), tracks, signalling, and scenery (roads, buildings, vehicles, model figures, lights, and natural features such as streams, hills and canyons.)
The size of the engines depends on the scale being used and can vary from around 700 mm (28") tall for the largest ridable live steam scales such as 1:8, down to matchbox size for the smallest ones in Z-scale (1:220). A typical HO (1:87) engine is around 50 mm (2") tall, and 100 mm to 300 mm (4" to 12") in length. The six most popular scales used are: G scale, Gauge 1, O scale, H0 scale (in Britain, the similarly sized 00 is used), TT scale, and N scale (1:160), although there is growing interest in Z scale. H0 scale is the single most popular scale of model railroad. Popular narrow-gauge scales include HOn3 Scale and Nn3, which are the same scale as HO and N, except with a narrower spacing between the tracks (in these examples, a scale three feet instead of the 4'8.5" standard gauge).
The largest common scale is 1:8, with 1:4 sometimes used for park rides. G scale (Garden, 1:24 scale) is most popular for back yard modelling. It is easier to fit a G scale model into a garden landscape and still keep the scenery proportional to the size of the trains running through. [Gauge 1] and Gauge 3 are also popular for garden layouts. 0, H0 scale, and N scale are more often used indoors. Lionel trains in 0 scale (1:48 scale) are popular children's toys.
The words scale and gauge seem at first to be used interchangeably in model railways, but their meanings are different. Scale is the model's measurement as a proportion to the original, while gauge is the measurement between the two running rails of the track.
At first, model railways were not to scale. Manufacturers and hobbyists soon arrived at de facto standards for interchangeability, such as gauge, but trains were only a rough approximation to the real thing. See Normen Europäischer Modelleisenbahnen (NEM) and NMRA. Official scales for the various gauges were soon drawn up, but the scales were not at first at all rigidly followed, and were not necessarily correctly proportioned for the rail gauge chosen. O (zero) gauge trains, for instance, operate on track that is too widely spaced in the United States as the scale is accepted as 1:48 where as in Britain 0 gauge use a scale ratio of 43.5:1 or 7 mm/1 foot and the gauge is much near to correct. The British 00 standards operate on track that is significantly too narrow. (The 4 mm/1 foot scale on a 16.5 mm gauge corresponds to a track gauge of 4' 1 1/2", 7 inches under-sized). 16.5 mm gauge corresponds to 4'8.5" standard gauge when modelling in H0 (half zero) 3.5 mm/1 foot or 1:87. Most of the commercial scales also have standards that include wheel flanges that are too deep, wheel treads that are too wide, and rail tracks that are too large.
Later on, groups of modellers became dissatisfied with these inaccuracies, and developed finescale standards in which everything is correctly scaled. These are used by dedicated modellers but have not generally spread to mass-produced equipment in part because the inaccuracies and overscale properties of the commercial scales are necessary to ensure reliable operation in the hands of consumers as well as experts, and also to allow for shortcuts necessary for cost control. These finescale standards include the UK's P4, and the even finer S4, which use a set of track dimensions scaled from the prototype. This 4 mm:1ft modelling uses wheels 2 mm (or less) wide running on track with a gauge of 18.83 mm. Check-rail and wing-rail clearances are similarly accurate.
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