At a time when the Atlantic wheel arrangement was passâ on most railroads and the larger Pacific had become the new normal for passenger power, the Standard Railroad of the World entrusted its mainline expresses to a new class of E6s Atlantics. Following the mantra set down by an earlier Pennsy designer that "every pair of drivers adds complications of machinery and friction," Alfred W. Gibbs, the Pennsylvania Railroad's first and only Chief Mechanical Engineer, created a muscular Atlantic that could do the work of a Pacific of its time, cost less to build and operate, and easily topped 100 mph.
The "Big E" was made necessary and possible by two developments on the Pennsy: the advent of heavier steel passenger cars and the upgrading of its mainlines to handle greater locomotive axle loads. Gibbs honed his design over a period of four years, beginning with a 1910 prototype and two more examples that were scientifically tested on the road and in the Pennsy's state-of-the-art Altoona Test Plant. He borrowed the basic boiler design from the Pennsy H8 2-8-0 freight engine, which boasted the largest boiler used on a PRR engine up to that time. Innovations in the suspension system allowed the E6s to ride exceptionally smoothly, track well at high speeds, and produce less wear on the track than contemporary Pacifics.
Superheating, a relatively new invention that increased steam temperature by passing the steam back through the boiler, was employed on the second and third prototypes. Testing proved that superheating increased power by 30% while decreasing coal and water consumption versus the original E6 prototype - and added the "s" to the engine's class name, E6s.
Designed, tested and built in Altoona's Juniata Shops, the 80 production engines delivered in 1914 were truly a homegrown Pennsy product. In the words of famed Trains magazine editor David P. Morgan, "to the end of her days the E6 found few companions that could stay with her above 100 mph or equal her output of more than 1200 h.p. per driving axle." She was "a genuine classic that would last through two world wars and not be stilled until dieseldom."
Originally assigned to the most prestigious trains on the relatively level New York-Washington corridor and westward to Harrisburgh, the E6s' were bumped to lesser mainline runs in the 1920s by heavier K4s Pacifics. Later yet, they ran in local and commuter service on the Pennsy and subsidiaries Long Island Rail Road and Pennsylvania-Reading Seashore Lines. But to the end of steam, the big Atlantics remained a class act. David Morgan recalled an encounter with an elderly E6s as a small child on vacation in Cape May, NJ, where the fireman "explained with obvious pride the finer points of his classic racer. It was rather like being invited to pet Man o' War just before he was taken out on the track."