The K-5 class designated the heaviest Pacifics of the New York Central System. Built primarily for the subsidiaries, they were divided into the K-5 and K-5a subclasses. Numbers 4915 and 4917 were fitted with streamlining in 1936 designed by Henry Dreyfuss and called the “upside down bathtub”. These two pulled important express trains like the “Mercury” and became subclass K-5b.