As I understand it, the Doble “steam generator” was basically a form of forced draught water-tube boiler, sometimes of the single-tube (monotube) type. Water-tube boilers were/are the norm for stationary and marine applications, but never found favour for locomotive applications. Hybrid combinations of water-tube fireboxes coupled with normal fire-tube boilers were tried here and there, notably by the B&O, but were never attractive enough to justify their wider use.
For a passenger auto with its relatively small steam demand, a boiler or “steam generator” and control system could be made with sufficiently low thermal inertia that it could work on an “immediate demand” basis. Not really so with the Stephensonian-type locomotive boiler relying on exhaust steam to create the draft, and which was required to be a thermal reservoir capable of supplying steam beyond its production capability for short periods of time. I imagine that the thermal reservoir requirement may have explained the hybrid of water-tube firebox with fire-tube boiler; the latter would have provided the thermal reservoir.
The apparent water economy of the Doble system was I think due to its use of a condenser for water recovery. Condensing has been used on conventional steam locomotives, with some fleet-scale applications, the best known of which is probably the South African Railways (SAR) 25 class, which was built for operation over the non-electrified sections that traversed the dry interior region known as the Karoo. Where water was very scarce and other solutions were not readily available, condensing could be made to work, but it was probably too much hassle for use where it was not unavoidably necessary. In the SAR case, a combination of electrification and dieselization made the condensing steam locomotive redundant, and most of the 25 class fleet was converted to non-condensing form (25NC class).
Cheers,
