A few Words about Sn3½ Scale & Gauge
For the uninitiated, "Scale" reflects the proportion of reduction in relation to the prototype, ie HO Scale is modeled at 3.5mm= 1 foot (304.8mm) and its reduction ratio is 1:87
Where as, "Gauge" represents the track width eg HO Scale trains run on 16.5mm track, representing the "Uniform" or Standard Gauge (4'8½") This being the standard "HO" originating from USA and "00" in the UK
In Queensland we have a prototype gauge of 3'6", but if we model in HO SCALE, (1:87) which to be correct, means that we should use 12mm track to accurately (within the limits of what track is commercially available) represent the 3'6" gauge (in HO Scale).
However there are some modellers who do not use the 12mm track, as determined by the above, as they use the common 16.5mm track, as in most cases, they already have such, and possibly have other trains of other systems or countries which they can run without encountering gauge problems!
in a larger SCALE, namely S Scale, we can model in a different reduction ratio of 1:64, which produces a larger model, but runs on the same HO track that most modellers already have, due to the fact that 3' 6" scales back (almost) to 16.5mm, which is commercially available, HO Scale track.
The 16.5mm proportions out to 3'6" in S Scale
The modelling scale is known as Sn3½
The breakdown of the digits means:
(in this case it is in "feet", but has also been written in places as Sn1067, other varieties are Sn2, On2 or HOn3½)