My bet is on a supplemental water tank – see how small the tender waterspace is?
Note how this is out on the pilot deck where not only its mass but leverage effects aid in producing adhesion for the forward engine – not that it probably needs all that much in actual reality. If I were running this, I’d ensure that the cistern on the tender can be essentially fully emptied before the forward tank starts… this being similar to the pumped-auxiliary-tank approach desirable on Garratts.
It is not a feedwater heater. See the big filler opening, and the lack of exhaust-steam piping. My guess is that there wasn’t enough exhaust-steam energy for proper drafting, let alone additional feed heating… But at least someone appears to have been thinking.
Of course I was just being a smart aleck. My actual guess was an air tank, much like the massive air tanks on the front of PRR’s N1s 2-10-2’s. I guess auxiliary water is worth considering, but I’m not aware of a precedent for it.
In retrospect, I think you’re probably right. Look at those little tiny supports under the tank – those ‘wouldn’t hold water’ … ;-}
And that brings us to ‘what can we tell about operation of this locomotive from the presence of that tank’. Perhaps hump service?. The ability to charge up a trainline in minimum time, perhaps on long cuts of cars with indifferent hose and gladhand-seal leakage, or in cold weather. What did Frisco run that would require heavy coal-train-style blocks of equipment desultorily maintained?
We usually think of Mallets in over-the-road service or helper service; but they were very commonly used in hump service on such roads as NYC (and P&LE), RF&P, and lots of others.
The Frisco Mallets didn’t come with the air tank on the front porch as original equipment, Frisco apparently added them in the 1920’s when they were transferred to Alabama. According to “Frisco Power”, they were built to haul freight at moderate speeds on the rolling hills between Springfield and Thayer Missouri. Since the firebox was too small to make sufficient steam for their intended purpose, they were reassigned to Alabama, hauling coal until they were scrapped in the 1930’s. Lots of good pictures in the book, not as many online… but here is one without that monstrous air tank:
