The Pennsylvania Railroad (PRR) class I1ssteam locomotives were the largest class of 2-10-0 "Decapods" in the United States. From 1916 to 1923, 598 locomotives were produced (123 at Altoona Works and 475 at Baldwin Locomotive Works). They were the dominant freight locomotive on the system until World War II and remained in service until 1957. Nicknames for the type included Decs and Hippos, the latter due to their large boiler.
The I1s design was much larger than the 2-10-0 design that preceded it, taking advantage of the PRR's heavy trackage and high allowed axle load, with a wide, free-steaming boiler. Large cylinders enabled the I1s to apply that power to the rails. However, the large boiler limited the size of the driving wheels, which made it impossible to mount counterweights large enough to balance the piston thrusts. As a result, they were hard riding at anything but low speeds, prone to slipping, and unpopular with crews.[1] The locomotives were known for being very powerful, with one author describing them as "the holy terror of the PRR".
Subclass I1sa increased maximum steam cut-off to admit steam for 78% of the piston stroke (rather than the original 50%), boosting low speed tractive effort from 90,000 to 96,000 pounds-force (400 to 430 kN). There was no obvious external difference, except for a revised builders' plate and combination lever. The I1s locomotives were converted to I1sa during major overhauls; eventually, 489 were converted while 109 remained as-built.
