There likely are VERY few that have the Nelson Gray cars available. Since Micro-Trains purchased the molds, I would expect that their current 40' Gondola/Flat and Box car frame would be. I have dozens of imperfect frames I could dig up and send you 1 or 2 to test fit.
There was an article in Ztrack magazine a few years back with pictures (IIRC)
I Unzip'd the pix so that others can see, although it is only a look at a Gondola car. There is some noticeable warping.
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Jeff
SF Bay Area Z
a.k.a. 'The BAZ BoyZ'
Jeff Merrill BAZ BoyZ