Isn't the Challenger, aside from wheel arrangement, just as large or larger than the Big Boy? At least heavier?
Restoring a steam loco that large, even if stored under "good" conditions, will be expensive. I am wondering if some UP senior executive is thinking that, per million dollars spent, the Big Boy might be a better public relations and advertising buy than any other buy per million. If so, it might really happen, and then get stalled halfway through when that exec leaves or is fired.