How about an 0-4-4T return flue geared locomotive? [%-)] SP’s subsidiary Oregon & California Railroad had two of these, used during construction. They were “numbered” “A” and “B” respectively.
With a return flu, the boiler gasses pass through to the front and returned to the rear of the boiler via U-shaped flues. I presume this was an attempt toward efficiency by having the hot gasses pass through the boiler twice. I’d bet it created a maintenance nightmare.
My money would be on the beast being a woodburner, and the roof being very useful to keep the woodpile dry in an area that gets more rain in a week than Southern Nevada sees in a wet year.
From what I can see, the boiler front reminds me of a Scotch marine boiler, which was a return flue design. They weren’t any worse to maintain than a standard boiler, but I’ll bet having the final smokebox in the cab was, to put it mildly, cozy. (I wonder if they swiped the boiler off a small steam-powered watercraft.)
The critters were rebuilt from locomotives acquired from the Cascade Railroad, built by the Vulcan Iron Works in 1862 and 1866. After the Oregon & California, they went to the Oregon Central.
That ‘bulge’ is for the traction motor blower fan. The GP30 had two ducts under the walkways to route to cool air to the traction motors. Later EMD production used just the left hand ductwork to route the cooling air. This reduced the need for multiple blowers over each traction motor(actually reduced the need for a longer car body). The GP30 was the first central air system EMD road engine.
For your info, the cylinders are 6x12 and driver diameter is 34 inches. Having the loco jointed will allow for a far smaller turning radius than if built solid.