I'll let others talk about other scales/gauges – but in Gauge 3 (2.5" ) which you correctly describe as being 1:22.6 for British standard gauge railways – the rule of thumb is to use either 13.5mm or 17/32 to the foot – which is pretty near to being correct scale anyway.

I used to work using 13.5mm as the conversion factor myself – but prefer these days (drawing in CAD) – to draw any prototypes 'full size' – then scale (using 22.6) them down and make any sensible adjustments required (for materials/fixings etc). So it's not really a problem building a "scale" model these days – except of course that some things just don't scale!

Regards,

IanT

Edited By IanT on 31/05/2018 20:56:14

The main rail gauges go up by root 2 roughly (1.4x) (starting with G1 obviously) with adjustments to a nice round figure in inches and fractions. Some then have a metric round figure equivalent. The gauge is not initially made to a scale or ratio of full size.
Then the body of the model is made to a scale that about fits the gauge but is again adjusted to a good round figure for either the ratio or the scale factor. More than one ratio can be used for each gauge.

****The ratio used for the gauge is not the same as the ratio used for the body.***

Then some purists come along and don't like that so adjust either gauge or scale to match, but not always exactly so along comes another load of purists who do want it to match even better. (eg H0, 00, Proto4, Scale4 each with slight changes).

Some more people stuck in a few extras like gauge 2 which caught a cold big time or minority following like TT (3mm : 1ft) or manufacturers deliberately went different to lock in customers like LGB.

Narrow gauge gives the opportunity for a whole bunch more configurations.

Edited By Bazyle on 31/05/2018 23:25:32