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When planning a model railroad layout, model railroaders have to choose what scale they are going to be modeling. The ratio between real life size and model size is known as the scale. On an HO scale layout, or 1:87 scale, the scenery, rolling stock and buildings will be 1/87 of the real life, or prototype size.
The most popular model railroad scale today is HO Scale, or 1:87. The smaller scales are N scale (1:160) and Z scale (1:220) and are ideal scales to use where you have a limited amount of space in which to build your layout. On an N scale layout everything would be 1/160 the size of full scale, and on a Z scale layout, things would be 1/220 the size of full scale. The smaller scales call for better vision and hand eye coordination when assembling buildings and working on rolling stock.
S Scale (1:64), O scale (1:48) and G scale (1:24) make up the larger model railroad scales. O scale became very popular back in the 50's and 60's when Lionel was the household name for model railroading. It is still a very popular scale. G or Garden scale is very popular for outdoor model railroad layouts. G scale works well on outdoor layouts as it is fairly simple to keep the existing scenery in proportion to the rolling stock.
Garden Railroad Clubs are very popular and have sprouted up across the country in large numbers. Garden railroads are ideal for a husband and wife to share the same hobby. One can tend to the plants and the other the railroad. There is also a new breed of garden centers which make available plants and shrubs sized just right for the G scale Garden Railroads.
Contrary to popular belief, the terms gauge and scale do not mean the same thing. While scale refers to the ratio between model and real life size, gauge refers to the distance measured between the rails. Most scales are modeled using standard gauge track, which in real life, has a rail to rail measurement of 4 feet 8 and a half inches.
Some modelers prefer to model narrow gauge railroads, or 3 foot gauge. This is done by using a combination of gauges and scales. If you modeled narrow gauge in HO scale, you would be modeling HOn3, where the "n" means narrow gauge and the "3" means 3 foot gauge track. The 10.5mm gauge track one would use in HOn3 is narrower than if you were using HO scale track.
Narrow gauge can also be modeled in S scale, N scale and O scale, and these are called Sn3, Nn3 and On3. What differentiates these scales from their parent scales is the use of narrow gauge track. Rolling stock, buildings and scenery remain unchanged. Modelers can even model 30" gauge track and 2 foot gauge track. Some examples of this are On30 and On2.
There are also even larger scales such as 1:12, 1:8 and 1:4. These would be used on park rides and on live steam layouts where you actually ride on the rolling stock.