The Pennsylvania Railroad's class I1ssteam locomotives were the largest class of 2-10-0 "Decapods" in the United States, with 598 built 1916–1923 (Altoona: 123, Baldwin: 475). They were the dominant freight locomotive on the system until World War II, and they remained in service until the end of PRR steam in 1957. Nicknames for the type included Decs and Hippos, the latter due to the large boiler. Unlike smaller 2-10-0s that preceded them, the I1s design was huge, 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. Their power was undeniable, but they were not popular with the crews, for they were hard riding at all but low speeds. One author described them as the holy terror of the PRR. The large boiler limited the size of the driving wheels, which made it impossible to mount counterweights large enough to balance the piston thrusts. Their factor of adhesion is low, so they were prone to slipping.
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 of course the revised combination lever). I1s locomotives were converted to I1sa during major overhaul; eventually 489 were converted while 109 remained as built.
In 1923 PRR put engine 4358 on the Altoona test plant. The tests below were all stoker fired.
In two hours at 7.12 miles per hour (11.46 km/h) it consumed 8,129 pounds (3,687 kg) of coal (13,398 BTU/lb or 5,760 kJ/kg) and averaged 71,993 pounds (32,655 kg) of tractive effort at the driver rims, corresponding to 6.4% thermal efficiency.
In one hour at 14.24 miles per hour (22.92 km/h) it consumed 6,809 pounds (3,089 kg) of coal (12,682 BTU/lb or 5,452 kJ/kg) and averaged 63,263 pounds (28,696 kg) tractive effort, or 7.1% efficiency.
At 22.02 miles per hour (35.44 km/h) (40% cutoff, so not a maximum effort) it consumed 7,000 lb/h (3,200 kg/h) of 13,039 BTU/lb (5,606 kJ/kg) coal and averaged 43,515 pounds (19,738 kg) TE, for 7.1% efficiency.
At 21.36 miles per hour (34.38 km/h) at 50% cutoff it consumed 5,230 pounds (2,370 kg) of 13,372 BTU/lb (5,749 kJ/kg) coal in 30 minutes averaging 51,409 pounds (23,319 kg) TE, for an efficiency of just under 5.4%.