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 T-1 and Q-2 duplexes or the S-2 steam turbine; some of the tried-and-true K-4s 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, built by the PRR's Juniata Shops in 1923, 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.