Choice of Scale & Gauge
Scale. Gauge. Arrrgh. My head hurts.
Before I go on it would be good to clarify "Gauge" and "Scale". The gauge is the distance between the rails. The scale is size multiplier. The latter is, bizarrely, typically given in pan-dimensional units, 7 millimeters to 1 foot, for example. It doesn't help that the "letter" description: e.g. OO, HO, O, G, N etc. attempts to roll up gauge and scale - but in garden railways that really falls apart.
GaugeIt's simpler to start with gauge if only because there are fewer of them and a mm is a mm. The familiar indoor "OO/HO" train set runs on rails 16mm apart. The bigger "O" models run on rails 32mm apart.
"G" railways, designed for gardens, typically run on rails 45mm apart.
Real world, normal, rail tracks are 4ft 8inches apart. This is called "Standard Gauge" - anything narrower than this is called "Narrow Gauge". There is one Standard Gauge; there are many and various Narrow Gauges.
ScaleNow we come to scale. For a scale of 7mm:1ft, the standard 4ft 8inches becomes 32mm. Voila! you have O Gauge. However, this is the one and only "high street" scale/gauge where this actually works!
HO ought to be "Half O" i.e. 3.5mm:ft and thus 16mm gauge, but, for messy historical reasons, the fudge that is OO/HO is "OO" trains (at 4mm:ft) on HO track (at 3.5mm:ft). So actually, your Hornby set is narrow gauge (herein lies many years of argument and debate which I shall quickly ignore and move on...)
G is a mess. Originally designed to be 13.5mm/ft practically all compliance to a scale has been abandoned by different manufacturers. The net effect is that all "G" rolling stock is roughly of "a certain size" regardless of what size the original model was. The scale is fudged to suit. Grrr.
Gauge 1 is the accurate scaling of Standard Gauge to the 45mm model gauge. These trains are gorgeous accurate models and big enough to sensibly power with live steam. They are also expensive. Heart-attack-inducingly expensive.
16mm - yes a naming lurch but that's because here the scale is important but the gauge not - just like real life narrow gauge railways. This normally is run on 32 or 45 mm rail. For 32mm rail, this is akin to Welsh mountain railways (e.g. Festiniog) that run on 2ft gauge track.
Gauge 3 is the natural doubling of O Gauge to a model gauge of 64mm. Remember that doubling in one dimension results in a model that's 8 times the size of it's O equivalent, so these are something like G in size, but scaled for Standard Gauge. No idea whatever happenned to "Gauge 2".
The typical consideration for garden railways is either 32 or 45 mm gauge and thus one of popular-but-random G stock on 45mm; O gauge on 32mm; or a scaled 16mm:ft Narrow Gauge railway on either 32 (2ft) or 45 (metre) gauge track.
My various desires point to one of the middle-of-the-range scales. Gauge 1 was out on budget, sadly. The erratic scaling of G is just infuriating (but is most of what I'm running now - just to have something to run). O Gauge (7mm:1ft) wasn't really physically big enough, very expensive and not enough of a step change from our indoor 4mm and 2mm layouts.
We did briefly consider a 5" gauge sit-on-able railway as the garden is just big enough, but it would have been very limited in scope for the expense, and quite intrusive.
My desire for a consistent scale hadn't really materialised by the time we started. A bit of a mistake perhaps but it wasn't really until the various bits of eBay'd stock came together that the inconsistencies really became visible.
The choice between 45mm and 32mm gauge was a very close one. We'd seem both in action on very different garden railways. Using 32mm would have saved about 10% on the track cost. In the end 45mm won on the basis of ease of availability, physical tolerance of 'leaves on the line', the non-exclusion of Gauge 1, and the gut feeling that, while historically accurate, 32mm gauge track just looked too narrow in model form.