I have a rough idea of the diagram for a watertube boiler in a steam locomotive. Unfortunately, all of the boiler diagrams I've seen show firetube boilers for a locomotive but watertube boilers only for stationary installations. Where can I find a diagram for a watertube boiler in a locomotive?
Maybe not a good enough diagram for you, but you'd think Railway Age or Rwy Mech Engr would have something. Hathitrust.org has most all of the latter, except you have to search for its new title, Railway Locomotives and Cars.
Look for articles on the D&H high pressure experimentals in Railway Mechanical Engineer. The Internet Archive has a nearly complete set of the issues on-line.
For the D&H locomotives, it appears that the water tubes were used in the firebox with a pair of steam drums on top. There appears to be a small firetube section between the firebox and smokebox that would effectively be the feedwater heater.
Keep in mind that a great many "watertube" boilers for American locomotives were actually watertube fireboxes (replacing the troublesome staybolted-sheet construction with so many issues) appended to a more-or-less conventional firetube convection section.
The boiler on Baldwin 60000 was famously of this construction. So were the various B&O locomotives with Emerson boilers (or, more correctly, Emerson fireboxes).
True watertube locomotive boilers have a troubled history. The Brotan boiler was among the more successful (but in European practice); I suspect its actual heating surface and tolerance of feedwater TDS was not considered adequate for much United States practice. Britain tried hard with the Yarrow adapted marine design in Gresley's 'hush-hush' locomotive 10000, and the list of reasons why this boiler 'failed to thrive' is illustrative of some of the technical troubles with watertube designs for general railroad service.
The 'generators' used for a number of steam railcars, and for Besler's and Doble's developments, were of course "watertube" boilers, and many of them were once-through. The greatest success here was not in locomotive or railcar power boilers, but in steam generators for electric and diesel locomotives.
A good effort was made with the Steamotive development in the Thirties, first at 1200psi and then 1500psi with well-thought-out mandatory condensation and turboelectric drive. Supposedly the bugs reported on UP were worked out, or weren't showstopping, on GN during wartime -- but GE's test engines were scrapped and to my knowledge no further use of the Steamotive plant was made.
The 600psi B&W boiler on the N&W TE-1 was a valuable attempt at a true watertube plant -- it was just too small for what was needed, and I have been assured that it couldn't effectively be 'scaled up' to where the output horsepower would justify the additional cost, complexity, and operating issues. The boiler on Bulleid's 'Turf Burner' used similar construction, including the traveling-grate firing, and the boiler was not remarked on as being a 'weak point' in that locomotive's design or operation. It just was too late to matter.
Something that did not work on large locomotives was the idea of a Benson or once-through boiler. Part of what is now locomotive 18 201 was a locomotive using such a boiler -- it was apparently not anyone's idea of a success, for the very short time it appears to have operated without failure.
The SACA-style LaMont firebox (which uses recirculating forced circulation in a waterwall radiant section) is not a once-through system either in the monotube or Benson style of circulation, and it can be combined with effective Cunningham circulation from a conventional firetube convection section. It should be noted that if the convection 'boiler' is fully populated with tubes and flues, it is easily capable of containing pressures as high as necessary on a road locomotive, eliminating the cost, complexity, and maintenance problems of a watertube convection section.
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