Disappointingly before my time with Z scale. I'd stand in line too. Maybe someday Rob and Hans will release round 2, if they were that popular. Can't even find one on fleabay...you know they're coveted if you can't find one there.
Not entirely sure, but there may be one, or a Challenger, in the Omaha rail museum. Don't drive thru Omaha much any more, but if I do I'd stop in for sure. In my neck of the woods there is a restored Nickel Plate 4-6-2 running and another being restored to running condition. Huge task. Can't imagine what it would take to get a Big Boy running...monumental. Just seeing/hearing the 4-6-2 come by is awesome...imagine more than 2X the size. Wow.