Here's 4-6-6-4 #5125 awaiting duty: (Yesteryear Depot pic) Steam on NP doesn't get much bigger than this!!
Those big NP artics have always been some of my favourites, they always look fearsome with those smokebox mounted pumps! Is this a Z5?
I always liked the Z3s. A smaller big engine that was limited to around 600 tons on the Wallace branch.
I'd also be interested in knowing a location. The background has me thinking possibly Missoula. Otherwise, no place else quite seems to quite fit. At least not on the west side of Montana. However, a 4500 series should have been a Class Z-4. Boxcab E50