The PRR E6s was not the most numerous 4-4-2 "Atlantic" built by the railroad, but it was the final and most successful Atlantic on the PRR. The E6 was first designed in 1910, and in extensive testing, the E6 prototype consistently outperformed the K2. In 1912, the prototype was super-heated and also had her cylinders enlarged to 23". The locomotive became the E6s, and the PRR built 80 E6s locomotives in 1914. Most of the engines were assigned to New York - Philadelphia - Washington passenger service. They were excellent locomotives, with several lasting until the end of steam on the PRR.