MODEL RAILROADING with LAURIE GREEN 

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WHAT IS 'GAUGE', SCALE' & 'NARROW GAUGE'? 
For those of you who are new to the hobby of model railroads, or are just surfing through this site, some of the terms used in our hobby can be very confusing. Especially the terms like 'GAUGE' ,"NARROW GAUGE" and "SCALE". What do these terms mean?
WHAT IS 'GAUGE'? In railroad terms, the word "GAUGE" means the distance between the two rails. Out in the real world, most railroads that you will see have a distance between the rails of 4'8¼". This is termed "STANDARD GAUGE". There are some slight variations of this width, like in my home state of Victoria, where the gauge is just over 5 feet.. But mostly these railroads are about this width. The term "NARROW GAUGE" is used to denote any track gauge whose distance between the rails is less than "STANDARD GAUGE". In Australia, most of the narrow gauge lines have or had a track gauge of 2'6" or 3'6", with a few odd ones here and there. In America, narrow gauge lines were mostly 3'0" in the western and middle states, with 2'0" found on the east coast, as well as 2'6" scattered across the country. In Europe, gauges around 1 metre were very common. Some industrial lines got down to about 18" gauge. WHAT IS 'SCALE'? The term "SCALE" denotes what proportion our model is to the real life object that the model is based on. Any object that we model, whether it be a train, structure or a piece of nature such as a tree, is built to scale. For an example, the scale that I model in is called 'O' scale, which has a proportion to the real world of 1:48, or 1/4" to the foot. That means my models are 48 times smaller than the real thing. If you are modeling in 'H0' scale, it's proportion to the real world is 1 to 87, or 87 times smaller than the real world. There are many different modeling scales used by modelers throughout the world, and the most popular are listed and displayed below. The locomotives below start on the left with Z scale (1:220), then N scale (1:160), H0 scale (1:87), S scale (1:64) and finishing with O scale (1:48) on the right. 
SCALE and GAUGE combined Modelers who model STANDARD GAUGE denote that they work in 'N' scale, 'H0' Scale, 'O' scale etc, but those of us who model NARROW GAUGE need a way to denote both the scale and gauge that we model in. To do this we have what looks like a mathematical formula which denotes the scale, the fact that it's narrow gauge and what track gauge our trains run on.To denote the scale we work in, we narrow gaugers use the same code as the standard gaugers, i.e. 'N','H0', 'O' etc., however we add a lower case 'n', which denotes that our trains run on narrow gauge track. Finally, we add the numerical width of the gauge that our trains run on. This could be 30, which means the track gauge is 2'6" or 30 inches, or 3, which means the track gauge is 3 foot or 36 inches. In my case, I model in 'O' scale (1/4" to the foot), using trains that run on a track gauge of 3 feet. I thus say that I model in 'On3'. If I was modelling in H0 scale (1/87th) with a track gauge of 30 inches, I would say that I model in 'H0n30' or 'H0n2½'. I hope this goes some way to explaining what the terms narrow gauge, scale and gauge mean in the hobby of model railroads.

SCALE CONVERSION RATIOS 
ratio  G scale  No.1 scale  O scale  S scale  OO scale  H0 scale  N scale  Z scale  
1:24  G scale  100%  133%  200%  267%  317%  363%  667%  917% 
1:32  No.1 scale  75%  100%  150%  200%  238%  272%  500%  688% 
1:48  O scale  50%  67%  100%  133%  158%  181%  333%  458% 
1:64  S scale  37%  50%  75%  100%  119%  136%  250%  344% 
1:76  OO scale  32%  42%  63%  84%  100%  115%  211%  289% 
1:87  H0 scale  28%  37%  55%  73%  87%  100%  184%  253% 
1:160  N scale  15%  20%  30%  40%  48%  54%  100%  138% 
1:220  Z scale  11%  15%  22%  29%  35%  40%  73%  100% 