BRIAN FAYLE’S SCALE/GAUGE EXPLANATION
From my Arches handout:
Gauge is the distance between the rails on a railway. A Narrow Gauge railway is one that is built to a gauge that is less than the so-called Standard Gauge of 4’ foot 8 1/2 inches. This gauge is found throughout the world. Many different narrow gauges were used around the world. In the UK there were a number of lines, especially in Wales, that were in the 2 foot range and most of these are still active. Narrower gauges in the 18 - 15 inch range were found in many industrial lines serving mines, quarries, sandpits etc..
In our context scale refers to the proportion in size of the real thing that a model is made to with the prototype/original being 1:1 scale. The common North American railroad modelling scale is HO at 1:87 scale and 3.5 mm to the foot. For a variety of reasons the common British railway modelling scale is OO at 1:72 and 4mm to the foot. North American O scale is 1:48 and 1/4 inch to the foot whilst British O scale is 1:43.5 and 7mm to the foot. Confusing isn’t it!
The standard track gauge used in modelling to HO and OO scales is 16.5mm. In both O scales it is 32mm and for N scale it is 9mm. The term ‘HO gauge’ is commonly used to describe modelling to HO scale on 16.5mm track. When modelling narrow gauge lines this terminology leads to confusion. For instance, the wider of the two circuits on Arches uses HO gauge track but as the modelling is to 1:43.5 O scale the 16.5 mm gauge represents 2 foot 3 inches not 4 foot 8 1/2 inches. Similarly the narrower circuit uses N gauge track but this represents a 15 inch gauge, not 4 foot 8 1/2 inches, as the modelling is to 1:43.5 scale not the N scale of 1:160! The track used is ready-made Peco narrow gauge O16.5/On30 and OO9/HOn30. You will notice that the ties are spaced further apart than they are on standard OO/HO or N track.
From the Red Fox handout:
What exactly is Gn15?
15 inch gauge railways can be found all over the world. They are often hidden away working in factories, peat bogs, agriculture, mines, gravel pits and such like. There were companies producing motive power and stock on a commercial basis but there were an awful lot of “home grown” items as well in use. A freelancers delight.
The "G" means G scale as popularised by the German firm LGB for their garden railway system. This is nominally 1/2" = 1 foot. (13.5mm to be exact). "Gn15" is American-style notification, telling us that models in this scale use the same scale as standard 'G scale' garden railways (between 1:20.3 and 1:29), the "n" tells us that it's a scale used on narrow gauge railways and the "15" that the scale is intended to represent 15 inch gauge prototypes. It works out that at the scale of 1/2" to the foot, fifteen inch gauge comes out to be as near as makes no difference 16.5mm which is standard HO scale track. Alternative notations would be G16.5 (G scale using 16.5mm gauge track) and Gn11⁄4 (G scale representing a 1'3" prototype gauge). Gn15 seems the easiest of the three to type, say and remember!
All this means is that you get the dual benefits of working in a big scale, with plenty of detailing possible, in a very small space and bring garden railroading indoors.