To maximize the pulling power of ten-coupled freight locomotives, the PRR made use of its high permitted axle load and returned to the 2-10-0 wheel arrangement in 1916. The lack of a trailing axle did not allow for large drivers, but increased the starting tractive power in service with heavy, slow drag freight trains. They were nicknamed “Decs” for their wheel arrangement “Decapod” or “Hippos” for their unusually large boiler.

To maximize pulling power and efficiency, a high boiler pressure of 250 psi and huge, long-stroke cylinders were chosen. However, in order to limit steam consumption when starting, the valve cutoff was limited from the usual 90 to 50 percent. In order to be able to run through small curve radii, only the first and last drivers had wheel flanges. 

The first 123 locomotives were built between 1916 and 1919 by the PRR workshops in Altoona. Another 475 followed from Baldwin in 1922. While Altoona used a Type A Schmidt superheater, Baldwin installed a larger Type E superheater. When it was found that the larger superheater area did not have the desired effect, the second batch from 1929 was built with a smaller superheater. 

No. 6280 in June 1937 as helper in front of a special train pulled by K4s No. 3845

collection Taylor Rush

At the beginning of the 1930s, most I1s were rebuilt to I1sa, with the cutoff being increased again to 78 percent. Their retirement took place between 1950 and 1957. Only 4483 was preserved and was subsequently exhibited at the Westinghouse Air Brake Company for a long time. In 1982 it was purchased by the Western New York Railway Historical Society and is now located in Hamburg, New York.