The PRR's L1s class was a Mikado that received the same boiler as the K4s class Pacific. Despite the great performance of this boiler with the Belpaire firebox, these locomotives were not equipped with a stoker ex works and many of them were not retrofitted later. They were considered very successful all-rounders and, apart from the USRA designs and after the PRR class I1s, achieved the second largest number of any class of steam locomotives built for an American railway company.

A total of 344 were built at the PRR's Juniata shops in Altoona, 205 at Baldwin and 25 at Lima. However, it was not long before they were replaced in some roles by 2-10-0 and 4-8-2 locomotives. In the 1930s, many were also put into storage due to the economic crisis and the electrification of an important line. These came back into the picture during World War II when their services were urgently needed. Number 520 still exists today and is on display at the Railroad Museum of Pennsylvania in Strasburg.