narrow gauge model railroads probably has to do with two main factors...
One is that this smaller gauge track allows you to put more of the larger scale railroad equipment and operations in smaller spaces. The trains will be able to negotiate tighter curves than they would be able to otherwise.
The second is that more companies are making more lines of narrow gauge locomotives and rail cars than ever before. The advertising for these models is increasingly prevalent in the model railroad literature, thereby promoting even greater popularity.
For example, if you like the size of HO equipment and structures, but your space is limited (and whose isn't when it comes to building an empire), and particularly if you like rugged mountainous terrain with small trains winding through and around mountains, valleys and gulleys and/or if you like logging railroads, you might want to consider building a layout using HOn30 scale, where basically you have HO trains running on N scale-sized track. If you are more interested in long straight runs of passenger trains, you would probably choose standard gauge instead.
The naming of narrow gauge scales is a little confusing at first. You have to remember that the lower case n stands for narrow gauge, not N-scale.
Also, remember the difference between the words gauge (the measurement or distance between the rails) and scale (the ratio of the size of the trains and structures to the prototype).
So what does the designation HOn30 or On3 mean?
The initial letters refer to the scale size of the locomotives, rail cars, structures, scenery - everything except the track.
The numbers at the end of the designation stand for the distance between the rails. For example, if the number is 30, that means there are 30 scale-inches between the rails, instead of the usual standard 56-1/2 inches. Or, if the number is 3 or 3-1/2, this means there will be 3 scale-feet or 3-1/2 scale-feet respectively between the rails.
So, the definition of the following scales would be as follows:
On30 - O scale trains running on rails that are 30 scale-inches wide (HO track)
On3-1/2 - O scale trains on rails that are 42 scale-inches (3-1/2 scale-feet) wide (S scale track)
HOn30 - HO equipment on 30 scale-inch wide rails - N scale track (Same as HOn2-1/2)
HOn3-1/2 - HO trains on 3-1/2 scale-feet wide rails (TT scale track)
Nn3 - N scale trains running on rails that are 3 scale-feet, or 36 scale-inches, wide (basically, Z-scale track)
Sn3-1/2 or Sn42 - S scale equipment on 42 scale-inch wide rails (HO track)
What is a scale-inch or scale-foot?
A scale-inch is the actual measurement in a model that corresponds proportionately to the prototype. For example, one scale-inch in N scale is actually 1/160 of a real inch. In HO scale, one scale-foot would equal approximately 1/80th of a real foot.
The availability of these models has previously been a limiting factor to building a layout of this type, but no more. Supplies for all of the above scales are increasing in the marketplace. They may not be available in your local hobby shop yet, but they are easily found on the Internet and can be ordered through your local hobby shop, or ordered online by yourself.
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So, if you are looking for something a little different to model than just the regular standard scale, if you like the idea of running larger scales in smaller spaces, short winding trains in mountain areas, logging railroads or other short line railroads, you may want to consider this gauge. Many model railroaders are moving in this direction.
by Malcolm Furlow.
Understanding S Scale - All about S scale including narrow gauge.
From "Narrow Gauge" to "Model Train Scales"