You made me take them out and measure a little. 
That's it, I'm sure. They seem to measure about .308" back to back at the flange and the S-4.2 standard has a max of .303" and a min of .297". Then when you go to the track standard S-3.2 and find that the flangeway target is .028" with a max of .030" and a min of .027" you see it. But that also raises the issue of the mentioned new metal wheel flange thickness measurement of .028" and if true, that's likely to be trouble or at least very finicky as it consumes all tolerance and even .001" tight if the flangeway was at the minimum. Leaves no room for tolerance in the trackwork OR the wheel gauging.
Just to follow up on this. I just remeasured the flange thickness at multiple locations on multiple wheelsets in the pack (the MT 33" metal wheels). They are definitely .028" thick. I did it with a caliper, and although the flange thickness slightly tapers as it nears the outer edge, it makes very little difference to the measurement. The thinnest were .026" at the edge, but still .028" in the middle of the flange.
