When the first G5s rolled out of the Juniata shops in 1923, the Pennsylvania Railroad hadn't built a 4-6-0 in more than two decades. The reigning queens of mainline passenger service were high-speed E6s Atlantics and K4s Pacifics; lesser duties like commuter runs were delegated to hand-me-down locomotives serving out their last years before retirement. In the early 1920s, however, the need for secondary passenger power outstripped the supply, and the Pennsy found itself in need of a new commuter engine.
In response, its Mechanical Engineer William F. Kiesel, Jr. took the boiler from an E6s Atlantic and designed one of the largest and most powerful ten-wheelers ever built. Smaller drive wheels than an Atlantic and the lack of a trailing truck concentrated more engine weight on the drivers and produced an engine with great power and acceleration but a lower top speed - ideal qualities for the constant stop-and-start duties of a commuter engine. Like the I1s Decapod, the G5s was infamous among enginemen as a rough-riding steed; famed locomotive historian Alvin Staufer stated bluntly, "Riding qualities were sacrificed for power when Pennsy designed these G5s Ten Wheelers."
Although the ninety G5s' served all over the Pennsy, the greatest concentrations were found around Chicago, Pittsburgh - where they were nicknamed "Pittsburgh commuter engines" - and in New Jersey. Pennsy subsidiary Long Island Railroad owned an additional 31 G5s engines that were a familiar sight to New York City commuters.
In his book Pennsy Power, Staufer also noted that "The G5s at times assumed the role of backwoods branchline meanderer." He cites an example of a G5s-led milk train that ran 145 miles daily through rural Pennsylvania, trailing an old combine at the end for the occasional passenger - which sometimes included a hunter that the crew would let off at a clearing in the woods. Fan trips as early as the 1930s also were headed by ten-wheelers: "'Off the Beaten Track' excursions covering branch lines frequently drew G5s power, since larger engines were prohibited." Work trains were another assignment too menial for mainline power but fine for the versatile G5s, which often elicited adjectives such as "gutsy," "squat," "tough," or "husky."