Introduced in 1925, the 75 locomotives of the class became the principal motive power for all major expresses, and accelerated long-distance passenger timetables leading to new levels of service in the pre World War II period. They were the mainstay of passenger expresses for over 20 years before the advent of the 38 class. The class was used extensively for performance testing, and thus the development and trial of a number of technical improvements.