NWSL actually do a HOn3 26" diameter wheel which scales out at 36.4" in TT although the axle is even longer than the NEM TT ones. Looking closer to home the 3mm Society have some Fine Scale standards which are only slightly wider than this (1.65 - 1.75mm or 0.065"-0.069") and includes dimensions for 12mm gauge although none are currently ...

While the most significant standardised dimension of a model railway scale is the gauge, a typical scale standard covers many more aspects of model railways and defines scale-specific dimensions for items like catenary, rolling stock wheels, loading gauge, curve radii and grades for slopes, for instance.

The aim of this standard is to provide a set of practical dimensions that will give the model railway builder track and wheels that are compatible with the majority of commercially available models and

“wheels meet NMRA and G1MRA stan-dards” or “back-to-back spacing is nar-rower than standard.” That means we’ve checked measurements on the wheels of a product and compared them to published standards. If there’s anything grossly out of line with those standards, it could lead to difficulties in running on our rail-roads.

In the new approved 2/2010 S-4.3 (old "Hi-Rail") standard "wheels with deep flanges": click here for the 4.3 standard (Note that the large scale standards are on the last 2 pages) back to back gauge is 1.575" +.019, -.015, or a minimum of 1.560" and a maximum of 1.594"