In 1937, Loewy established a relationship with the Pennsylvania Railroad, and his most notable designs for the firm involved some of their passenger locomotives. He designed a streamlined shroud for K4s Pacific #3768 to haul the newly redesigned 1938 Broadway Limited. He followed by styling the experimental S1 locomotive, as well as the T1 class. In 1940, he designed a simplified version of the streamlined shroud for another four K4s. In 1942, he designed the streamlined shroud for the experimental duplex engine Q1 which was his last work of streamlining PRR's steam engine.