Märklin might do modestly well if it offered British-outline EMU stock - then the 3-rail system might be justified!
At the moment, the HO Gresley A3 is probably just a one-off, a bit like the Rivarossi "Royal Scot" or the Fleischmann "Warship". Not meant to lead to anything else, but useful to compare to Continental outline stock for an accurate idea as to relative size.
You can see from the CAD and test-shot model that Märklin have applied their traditionally robust standards (not greatly changed from what they were doing in the 1980s) , so the valve gear links and pivots have been made overscale whilst the wheels have massive flanges.
No, that was a Great Western "small Prairie" which also went to Poland and helped to run regular service on the line from Wolstyn.