The Pennsylvania Railroad DD1 was a class of boxcab electric locomotives built by the Pennsylvania Railroad. The locomotives were developed as part of the railroad's New York Tunnel Extension, which built the original Pennsylvania Station in New York City and linked it to New Jersey via the North River Tunnels. The Pennsylvania built a total of 66 locomotives in its Altoona Works; they operated in semi-permanently coupled pairs. Westinghouse supplied the electrical equipment. The first locomotives entered service in 1910, with the opening of Pennsylvania Station. They operated between Manhattan Transfer and Pennsylvania Station, and from there to the coach yards at Sunnyside Yard in Queens, New York. With the arrival of the Class L5 locomotives in 1924, some DD1s moved to the Pennsylvania-owned Long Island Rail Road (LIRR), which had substantial electrified commuter rail operations. The conversion of the New York–Philadelphia main line to alternating current in the 1930s saw the remainder of the DD1s scrapped or transferred to the LIRR. One pair, Nos. 3936 and 3937, is preserved at the Railroad Museum of Pennsylvania and is listed on the National Register of Historic Places. Design Each semi-permanently coupled pair had a length of and weighed . DD1-class locomotives were nearly always operated as a pair—never individually and rarely as two pairs in a double-heading configuration. The PRR classed their 4-4-0 locomotives as class D, and the DD1 was essentially two 4-4-0 locomotives coupled back to back, resulting in the new class, DD. Each pair was assigned a single "Electrified Zone Number" (EZN); the EZN simplified train dispatching for each pair of DD1s while their original individual serial numbers were used for the mechanical records of the locomotives. Each locomotive had its own Westinghouse 315-A, direct current, commutating pole, electric motors within a monocoque cab. The motors had a continuous power rating of at , and could produce up to at for no more than an hour. Their top speed was , but PRR/LIRR timetables had a speed limit of . The motors were connected to the two drivers via a jackshaft and coupling rods. The design of the DD1 served as a transition between steam locomotives and modern electric locomotives. Despite their ungainly appearance, DD1s ran "quietly and smoothly...with no appreciable rod clanking", and had a very low maintenance cost. DD1 locomotives operated off 650 volt direct current from a third rail. History The first DD1-class of locomotives