Up until the PCC car and the “new” Pacific and Mallet and Consolidation all this year (Kader anyone?), the Aristo locos have always been close to proper gage, but under spec on back to back.
Since back to back plus the 2 flange thicknesses = wheel gage, you cannot have gage and back to back right if the flange is too thick.
For years this has been the situation, and now I will narrow to just the steam locos (their wheel contour is different from the diesels)
Now, there is a “new” wheel attachment system, which consists of a bushing pressed into the wheel. This alone should not force any differences, but now the back to back is in spec and the gage is out.
The wheel contours are as they have been for years, with an overly thick flange, approximately 3.23 mm… (spec is about 1.5 mm or less)…
Well, it’s simple math to see if you have back to back in spec (let’s use the NMRA standard of 40.1 mm)…
So if you have flanges in spec, take the b-b for 40.1, add in the 2 flanges (1.5 * 2) and you get a wheel gage of 43.1 mm… pretty clear that 43.1 mm will fit on 45mm track.
Now, the new Aristo wheelsets have a back to back almost exactly 40mm (great) but you add on the 2 flanges (3.23 * 2) and you get just about 46.5 mm
Now 46.5 mm is wider than the rails… like I said, simple math.
So TAC, if you get a “new” Mallet, Pacific, Consolidation, or PCC car from Aristo, you will have this new situation.
Hope that helps explain why your existing stuff measures differently from the new stuff, using the same wheels.
Regards, Greg