Pennsylvania Railroad 5550 is a mainline duplex drive steam locomotive under construction in the United States. With an estimated completion by 2030, the locomotive will become the 53rd example of the Pennsylvania Railroad's T1 steam locomotive class and the only operational locomotive of its type, as well as the largest steam locomotive built in the United States since 1952. The estimated cost of PRR 5550 was originally $10 million, but an updated projected cost of $7 million was released with the acquisition of an existing long-haul tender from the Western New York Railway Historical Society in August 2017. Construction began in 2014 with the casting of the locomotive's keystone-shaped number plate. As of February 2024 the locomotive was 43% complete.

A duplex locomotive is a steam locomotive that divides the driving force on its wheels by using two pairs of cylinders rigidly mounted to a single locomotive frame; it is not an articulated locomotive. The concept was first used in France in 1863, but was particularly developed in the early 1930s by the Baldwin Locomotive Works, the largest commercial builder of steam locomotives in North America, under the supervision of its then chief engineer, Ralph P. Johnson.