The Pennsylvania Railroad's class E2, E3, E7steam locomotives were of the 4-4-2 "Atlantic" passenger type, frequently called light Atlantics after the introduction of the heavier E6 Atlantics. All were similar in size and boiler capacity but differed in firebox type, valves and valve gear and cylinder diameter. Classes E2 and E3 were built simultaneously.
Starting in 1916 a rebuilding program converted ninety class E2a, b, and c to class E7s by replacing slide valves with piston valves and increasing cylinder diameter from 20.5 to 22.5 inches (520 to 570 mm). Fourteen class E2 were similarly converted to class E7sa. Ninety class E2a,d, E3a,d were converted to class E3sd. These improvements allowed many of the engines to remain in active service into the 1930s.
In the first decade of the twentieth century classes E2 and E3 handled all of the fast passenger trains of the Pennsylvania Railroad. As train weights increased due to the switch to steel passenger cars and more cars per train, the “light” Atlantics were usually doubleheaded. Eventually, as Pacific class K2 and K3 became available, they were relegated to secondary service
