The Pennsylvania Railroad was noted for the standardization of its steam locomotives; its more than 4500 engines were limited to a small number of designs that shared parts with each other. For example, the K-4s Pacific used the same boiler as the L-1 Mikado. As a result, members of the same class originally tended to be look-alikes. During the 1930s and 1940s, however, the railroad applied some experimental features to selected members of its tried-and-true K-4s class. Here we see No. 5371, a 1924 graduate of the Juniata Shops, equipped with roller bearing main and side rods and web-spoke drivers. Additionally, the air reservoir has been relocated to rest above the pilot. The photo was taken at Trinway, Ohio in November 1941; possibly the rods were a study for the T-1 duplex, first built in 1942. The image, photographer not given, comes from Carl Weber.