The Pennsylvania Railroad was an innovator and experimenter with steam power through the World War II era and for a while afterward. Experimentation was not limited to the famous T1 and Q2 duplexes or the S2 steam turbine; some of the tried-and-true K4s Pacifics received experimental modifications as well. Three of them were fitted with different types of poppet valve gears, including No. 3847 shown here at East St. Louis in September 1950. This engine received the Franklin Type B rotary poppet valves, in which the cam is driven by a shaft that rotates via a gear suspended in the center of the main driver.

Poppet valves enabled a more precise timing of steam admission to the cylinders, and exhaust therefrom, than did conventional gears such as the Baker or Walschaert, and hence improved the performance of steam locomotives especially at higher speeds. However, they were difficult to maintain because they had smaller, less accessible parts and because most railroad shop personnel were not familiar with them. No. 3847 also received a front-end throttle and type HT stoker during its rebuild. Amazingly, No. 3847 appears several times in the Green Frog DVD The Golden Twilight of Postwar Steam, Part 1. This photo by an unidentified photographer is in my brother's collection.