I may have found a man who converts single reduction to double. I've sent him an email to ask and, as a bonus, he lives about 25 miles from me and belongs to the same model RR club that I do.
More later.
PS: got a box of goodies coming from Robert Grossman to restore a lot of Marx stuff.
More on that later, too.
PPS: anyone have a source for a chimney for a Marx tin station? I now have three, but one has no chimney.