Hey Martin, thanks!
Based on what you've said and from what I've seen about the wheel contours, I figured using the distance between the backs of the wheels would be the best choice. Problem is I don't know if that measurement really means anything, maybe measuring the distance between the outer surfaces of the wheel flanges on the tapered sides of the wheelset would be better. If that measurement was used as a standard, then at least we'd have a starting point. Different wheel contours would only raise/lower the height of the axle center line, making some cars ride a fraction higher/lower than others.
Surely the manufacturers have some criteria they go by when making wheels. They have to in order for them to run on all the different brands of track that are available. What we need to do is find out what that magic number is.
