One of the big issues that Steamtown has with motive power is that they have some odd requirements, and the overlap of those fall in a spot where the collection really doesn't have much coverage.
They have the stiff grade coming out of Scranton to go, well, pretty much anywhere. It rules out any of the low tractive effort engines, like Canadian Pacific 4-4-4 #2929 (high speed express engine with minimal tractive effort) or any of the really old teakettles, like the AC&Y 4-6-0, Brooks-Scanlon Co. 2-6-2, or the Norwood & St. Lawrence 2-6-0.
They need something with decent range, which eliminates any of the small industrial engines they have, like the E.J Lavino 0-6-0 saddletanker, the New Haven Trap Rock 0-4-0T, or the Groveton Paper Co. 2-4-2T. Those would also be incredibly slow. The Canadian National 4-6-4T commuter tank would be able to handle track speed and the 63" drivers means it has reasonable tractive effort, but the range would also be borderline.
They've also stated that, since they lack turning facilities at any of the further destinations, they would really like something with a rear trailing truck. It helps the engine track better when running tender first, it's easier on the drive wheel flanges, and it's less abusive to the head end crew. That rules out engines that would other wise have the power and the range, like Maine Central 2-8-0 #519, Lowville & Beaver River 2-8-0 #1923 (also, I've heard that one is in disastrous condition), Illinois Central 2-8-0 #790 or Rahway Valley 2-8-0 #15.
Well, CPR 4-6-2 #2317 and CNR 2-8-2s #3254 and #3377 all have decent power and range, as well as trailing trucks, and the #2317 and #3254 are proven capable of handling those trips. The #3254 has some pretty irreparable damage from a severe smash-up in her original life that was always problematic, but #3377 is reportedly in good shape and is the same class. The issue: they're Canadian. That was always a huge gripe, that Steamtown received $6 million dollars from the American federal government but was running Canadian steam.
So, what has power, range, a trailing truck and is American? You're left with Union Pacific 4-8-8-4 #4012, Nickel Plate Road 2-8-4 #759, Grand Trunk Western 4-8-2 #6039, Reading Company 4-8-4 #2124 and Boston & Maine #3713. The #4012 isn't even remotely feasible to operate, just based off of size and weight, and it'd cost a fortune to return to operation and operate. NKP #759 is in excellent mechanical shape and certainly has the power, but it barely fits the turntable and roundhouse, the NKP #765 folks say that #765 struggled to negotiate the curves and switches of the ready track, and Steamtown has said they don't want to operate a Van Sweringen Berkshire when there is already another two running. Reading #2124 is even larger and wouldn't fit the roundhouse or turntable at all, and they have a Reading T-1 practically running out of their backyard these days. And GTW #6039 looks great, would be unique in that there aren't any operating Mountains in the US, has the power and speed, and would fit the existing infrastructure, but it reportedly has scoured cylinders that would make it hideously expensive to return to operation.
So really, despite B&M #3713 being a less than ideal choice for their terrain, it's the best choice.