As built, the firebox had combustion chamber, thermic syphons. Powerful fleet of 20 passenger engines named after US Presidents (in order, beginning with #5300 President Washington). #5320 completed in Mt. Clare shops to watertube boiler design and fitted with Caprotti rotary valve gear. #5310 also converted to watertube boiler and streamlined for service on the Royal Blue.
Sagle, 1964, provides details on several rebuilding programs, but in summary they were as follows:
5306 became a P-7b in 1942 when she was fitted with a B&O-designed Type R superheater.
Four engines -- 5305, 5308-5309, 5318 -- were rebuilt as P-7c in 1944-45. These had their boilers remounted on solid-steel engine beds with integrally cast cylinders. Other changes included feedwater heaters, a front-deck shield around several pumps and pipes, and a lowered headlight. Two -- 5309 and 5318 -- were fitted with semi-watertube fireboxes. Both engine and tender trucks received roller bearings in 1951.
Similar rebuilds were wrought on several more P-7s. In 1946, four streamlined P-7d -- 5301-5304 -- were assigned to the Cincinnatian. These and the P-7e -- 5312, 5314-5317, 5319 -- also had the new frames of the P-7c. In addition, all axles were fitted with roller bearings. Four different firebox designs appeared on the P-7e: 5314 had the simplest rework, gaining thermic syphons and 2 arch tubes; 5315 got a syphon, 5 arch tubes, and a combustion chamber; 5317 had Nicholson circulators; and 5312, 5316, and 5319 received the semi-watertube fireboxes fitted to the 5309 and 5318.
The last P-7 retired in 1958.