To overcome the disadvantages of the class E1 Camelback Atlantics, the E2 was developed in a conventional arrangement. The elimination of the Wootten firebox reduced the grate area from 69 to 51 square feet and eliminated the combustion chamber, but increased the heating area from 2,320 to 2,430 square feet. The dimensions of the cylinders and wheels remained the same, and the same three-axle tender was used. Only the later variants received four-axle tenders.

After 82 of the class E2 built at Altoona, a total of 93 of the class E2a were built from 1902. They differed from the original model mainly in the Belpaire firebox and 32 of them came from ALCO-Schenectady. In 1903 and 1904, 70 E2b were built, which still had Stephenson valve gear, but now had piston instead of slide valves. Four of them formed the basis for the E28 and E29 class compound locomotives. Parallel to the E2b, 22 E2c were built, which again had slide valves. Finally, between 1906 and 1908, 32 E2d followed, which now had Walschaert valve gear and piston valves. In order to achieve a higher adhesive weight, the three carrying axles were loaded by a total of around five tons less than on the E2c.

A large number of locomotives of the various E2 variants were later superheated. This included the E2sd class, which compared to the E2d showed hardly any differences apart from the superheater. Others were rebuilt to different variants of the E3 and E7. These had larger cylinders and some smaller coupled wheels.