This is an area that often confuses people. Let's first take a look at the various 2mm & N gauge standards. Many years ago, one of the pioneers of HO/OO scale, A.R. Walkley, made an even smaller model of a Midland Railway 0-6-0 loco running on 3/8" gauge track. This would have been in about 1927. Assuming 3/8" to represent standard gauge 4ft .8½in., this gives a scale of two millimetres to represent one foot. Given that this was half of the scale dubbed as OO, the new gauge was named OOO.
This concept of using a specific dimension to represent one foot is a peculiarly English way of doing things. Most of the world uses scale ratios instead. In these terms, OOO/2mm scale is effectively 1:152 which is still in use today and promoted by the 2mm Scale Association, using a gauge of 9.42mm. While 2mm modelling has continued for many years on a scratchbuilding and limited trade support basis, N scale RTR was introduced by Arnold and in England (as OOO) by Lone Star, both using a gauge of 9mm. Arnold used a scale of 1:160, which is close to correct for 9mm gauge, while Lone Star continued with 1:152. When British N scale arrived, the primary supporting manufacturer, Peco, chose to ignore all previous experience and introduce yet another scale, 1:148, supposing (wrongly) to allow extra room for mechanisms in the smaller size of UK prototypes. What also happened around this time was that there were some appalling liberties taken with the scale dimensions of some models, which were forgiven "due the sub-miniature size of the models". Right, as long as the chimney is in front of the cab, who cares, eh? This attitude still persists today in some unenlightened N gauge circles, but fortunately, like the dinosaur, it's a dying breed.
So we now have three standards:
1:152 - 2mm scale (the old OOO standard)
1:148 - British N scale (2.0625mm to the foot)
1:160 - International N scale (1.906 mm to the foot)
Fortunately, N scale uses a standard (albeit hugely incorrect and ugly) gauge and wheel profile, so British and International N can be interchanged. The 2mm standards are very much finer and cannot be used on N gauge track. For standard gauge modelling, I prefer to use the 2mm standards as they are closer to scale, work well and look very much better.
Turning now to narrow gauge, the obvious move is to use Z gauge to represent 3ft. and/or metre gauge in N scale/2mm. Thus, the nominal 6.5mm gauge represents:
3ft 3in. in 2mm scale (just about spot on for metre gauge!)
3ft 1¾in. in 1:148 scale
3ft 5in. in 1:160 (very close to "Cape Gauge" of 3ft. 6in or 1067mm)
However, for most people, this is close enough for jazz, so 6.5mm is used to represent everything from 3ft to 3ft 6in.. For my own project, given that the narrow gauge is going to be a separate and somewhat freelance layout to the standard gauge, I shall work to 1:148 as it's then closer to a nominal 3ft gauge, being under 2 scale inches out. I can live with that!
Below are the published standards for Z gauge.
SO RAIL HEAD
B BACK TO BACK 5.25 mm min
G GAUGE 6.8 mm max
P RADIUS OF HEAD 0.1 mm
F FLANGE WIDTH 0.75 mm max
S SHOULDER 5.2 mm max
N WHEEL THICKNESS 1.55 mm max
T FLANGE THICKNESS 0.41 mm min - 0.45 mm max
H FLANGE DEPTH 0.6 mm min
K FLANGE FACE TO WHEEL BACK 5.9 mm max
C CHECK RAIL TO CHECK RAIL 5.9 mm min