The PRR and Reading operated both passenger and freight trains along their tracks in the Philadelphia region. Starting in 1915, both companies electrified their busiest lines to improve the efficiency of their passenger service. They used an overhead catenary trolley wire energized at 11,000 volts single-phase alternating current at 25 Hertz (Hz). The PRR electrified the Paoli line in 1915, the Chestnut Hill West line in 1918, and the Media/West Chester and Wilmington lines in 1928. Both railroads continued electrifying lines into the 1930s, replacing trains pulled by steam locomotives with electric multiple unit cars and locomotives. PRR electrification reached Trenton and Norristown in 1930. Reading began electrified operation in 1931 to West Trenton, Hatboro (extended to Warminster in 1974) and Doylestown; and in 1933 to Chestnut Hill East and Norristown. The notable exception was the line to Newtown, the Reading's only suburban route not electrified. While the PRR expanded electrification throughout the northeast (ultimately stretching from Washington, D.C. to New York City), the Reading never expanded electric lines beyond the Philadelphia commuter district.