I've seen it running. It's impressive.

As orpheline said, it was at a museum in Pomona, California, for decades. It's going back there October 12th and 13th (full schedule for the trip not available yet). If you're going to go see it, though, be prepared for an unreasonably large amount of traffic, even by Southern California standards. Like, an absolutely absurd amount of traffic. It's amazing how much attention the Big Boy gets.