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" ; 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 of 650 volt direct current from a third rail.
History
The first DD1-class of locomotives were introduced into regular passenger service on November 27, 1910 to operate in the North River Tunnels under the Hudson River. As steam locomotives were prohibited from entering the tunnels, the electric DD1 shuttled passengers from the Pennsylvania Railroad's Manhattan Transfer station in New Jersey and Pennsylvania Station in New York City. A total of 66 locomotives were constructed by the Pennsylvania Railroad's Juniata Shops in Altoona starting 1909; 3936 and 3937 were put into service in 1911. As the new L5s were being introduced in 1924, most DD1s were transferred to the Long Island Rail Road. Both 3936 and 3937 were shifted from mainline passenger duty, and hauled the empty passenger trains from Pennsylvania Station to the Sunnyside Yard. The Long Island Rail Road scrapped most of their DD1s from 1949 to 1951, and only two pairs remained in 1962. By 1978, Nos. 3936 and 3937 comprised the last existing DD1 and were donated to the Railroad Museum of Pennsylvania in Strasburg, Pennsylvania, by the Pennsylvania's successor Penn Central as part of a collection with twelve other historically significant locomotives. Both locomotives were jointly listed on the National Register of Historic Places on December 17, 1979.
