Why 5 Inch Gauge
Well first of all, what is meant by gauge? Gauge is the distance between the rails. Standard gauge track which nearly all trains run on throughout the world is 4ft 8.5 inches.
Using 5 inch gauge track for our miniature railway we can run trains that are almost 1 inch scale models. That is to say every 1 inch represents 1ft. In America they use 4 3/4 inch gauge track which is exactly 1 inch to the foot.
Even better, we can also model the Narrow Gauge. A common scale is 2.5 inches to the foot which can model Narrow Gauge prototype trains that ran (or still run) on 2ft gauge track. An example of this is the Tallylyn railway in Wales. This same scale is commonly used on 7.25 inch gauge track to represent the 3ft gauge prototype, and these locomotives are BIG!
5 inch gauge track was also chosen as its the most common ride on railway gauge used for garden railways and is the smallest gauge that is commonly used to carry passengers.
Why not 7.25 inch gauge?
It may be that in the future we will consider 7.25 inch gauge for our miniature railway or even dual gauge, but the cost of locomotives and equipment at this gauge is very expensive and even the smallest steam locomotive can weigh nearly a tonne and would require specialist equipment to move around.