The paragraph accompanying the photo of experimental 4-8-4 #800, class HS-1a, says the boiler produced steam at 800 lbs/sq.in and 25 lbs/sq.in. I take it these are the pressures of steam produced for use in the pistons: did it also have an extremely high pressure (ca. 1400 lbs/sq.in) closed cycle internal portion of the boiler, as did Cnadian Pacific's experimental 2-10-4? (Question also posted to New York Central forum.)
From Wikipedia:
[edit] The Schmidt SystemStuart
One way to avoid corrosion and scale problems at high pressure is to use distilled water, as is done in power stations[citation needed]. In fact you need to go further: dissolved gases such as oxygen and carbon dioxide also cause corrosion at high temperatures and pressures, and must be kept out. Most locomotives did not lug a condenser around with them, so there was no source of pure feed water. One solution was the Schmidt system[1]; this used a sealed ultra-high-pressure circuit that simply transferred heat to a high-pressure circuit, by means of heating coils inside a high-pressure boiler. If this latter is fed with ordinary water, scale may form on the outside of the heating coils, but it cannot cause overheating, as the ultra-HP tubes are quite capable of withstanding their internal steam temperature, though not the firebox flame temperature.
The sealed ultra-high-pressure circuit ran at between 1,200 and 1,600 psi (8.27 and 11.03 MPa), depending on the rate of firing. The HP boiler worked at approx 850 psi (5.86 MPa), and the low-pressure boiler at 200 to 250 psi (1.38 to 1.72 MPa). The UHP and HP boilers were of a water tube design, while the LP boiler was a fire tube boiler typical for steam locomotives. The LP cylinders were driven with a mixture of the HP cylinder exhaust and the LP boiler output. Both HP and LP boilers had superheaters.
The French PL241P, the German H17-206 and the British LMS 6399 Fury all used the Schmidt system, and were of basically similar design. The New York Central HS-1a and the Canadian 8000 also used the Schmidt system but were a size larger altogether- the 8000 weighed more than twice the Fury.