Strasburg Model Railroad Club displays all scales.

"Scale" is the relationship between the size of a model train and the real-life train it models.

Model trains come in a variety of sizes, or scales, from the very small model train to the larger garden railway variety. Though their sizes were originally described as their “gauge,” scale is now the accepted term. Choosing a favororite model train scale is really a matter of personal preference, experience, and budget. There are numerous variations of model train scales, but there are five that are the most common, making them the easiest to find. The simplest definition of a model train scale is the relevance of the reduced size to the original train being replicated.

Most Common model train scales: G - O - S - HO - N - Z

The most common model train scale is HO. HO model trains have an approximate ratio to the original train of 1:87. Variations of the HO model train scale include narrower versions of the same scale, which means the trains are the same scale, but with less space between the tracks’ rails. The HO model train scale is probably the easiest to find and the easiest to accessorize because many hobbyists use it. It is not so small that creating layouts is complicated, but it is large enough that it shows well.

The N model train scale is probably the second most popular size. N scale model trains are approximately 1:160. The N stands for nine, which is the distance in millimeters between the inside rails of the tracks used in this scale. They are smaller than the more popular HO scale, making them slightly more difficult to accessorize to scale, but the advantage to the N scale is using less space to create a complete layout.

Other common model train scales include O (1:48), S (1:64), G (1:24), and Z (1:220). G scale model trains are commonly seen in larger garden railway designs. Z scale model trains are very small, and while a complete layout could be easily achieved in confined spaces, people with large hands, poor eyesight, and physical challenges, such as arthritis, find it most difficult to work with this scale.

Since its introduction in the 30's, 1:64 or 3/16" to the foot scale model railroading has intrigued modelers of all ages. A big boost to the scale came in 1946 when A.C. Gilbert Co. marketed S Scale trains that operated on 2-rail track. This introduced millions of children and parents to the advantages of "S" over the diminutive HO or O-27, operating on three rail track with its un-prototypical stubby appearance.

Though there are other model train scales, these are the six most common scales used. Depending on the manufacturer, the actual ratio of replication may vary slightly. However, accessories and track will remain relatively standard to each scale.

The size of engines depends on the scale and can vary from 28 inches tall for the largest ridable live steam scales such as 1:8, down to matchbox size for the smallest in Z-scale (1:220). A typical HO (1:87) engine is 2 inches tall, and 4 to 12 inhes long. The most popular scales are: G scale, Gauge 1, O scale, H0 scale, TT scale, and N scale (1:160).There is growing interest in Z scale. H0 is the most popular. Popular narrow-gauge scales include HOn3 Scale and Nn3, which are the same as HO and N except with a narrower spacing between the tracks.

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 backyard modelling. It is easier to fit a G scale model into a garden and keep scenery proportional to the trains. [Gauge 1] and Gauge 3 are also popular for gardens. 0, H0 scale, and N scale are more often used indoors. Lionel trains in 0 scale (1:48 scale) are popular toys.

The words scale and gauge seem at first interchangeable but their meanings are different. Scale is the model's measurement as a proportion to the original, while gauge is the measurement between the rails.

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. Official scales for the gauges were drawn up but not at first rigidly followed and not necessarily correctly proportioned for the gauge chosen. O (zero) gauge trains, for instance, operate on track too widely spaced in the United States as the scale is accepted as 1:48 whereas in Britain 0 gauge uses a ratio of 43.5:1 or 7 mm/1 foot and the gauge is near to correct. British 00 standards operate on track significantly too narrow. The 4 mm/1 foot scale on a 16.5mm gauge corresponds to a track gauge of 4ft 1 1/2in, 7 inches undersized). 16.5 mm gauge corresponds to 4ft 8.5 standard gauge in H0 (half zero) 3.5 mm/1 foot or 1:87. Most commercial scales have standards that include wheel flanges that are too deep, wheel treads that are too wide, and rail tracks that are too large.

Later, modellers became dissatisfied with inaccuracies and developed standards in which everything is correctly scaled. These are used by modellers but have not spread to mass-production because the inaccuracies and overscale properties of the commercial scales ensure reliable operation and allow for shortcuts necessary for cost control. The finescale standards include the UK's P4, and the even finer S4, which uses track dimensions scaled from the prototype. This 4 mm:1ft modelling uses wheels 2mm or less wide running on track with a gauge of 18.83 mm. Check-rail and wing-rail clearances are similarly accurate.

A compromise of P4 and 00 is 'EM' which uses a gauge of 18.2mm with more generous tolerances than P4 for check clearances. It gives a better appearance than 00 though pointwork is not as close to reality as P4. It suits many where time and improved appearance are important.

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