With the completion of the large Penn Station in Manhattan and the tunnels towards Newark, electrical operations began on this route. To pull the passenger trains from Penn Station to the Manhattan Transfer Station about nine miles away, the class DD1 double locomotives were procured. These had the task of increasing the train weights on the sometimes steep gradients in the tunnels from 750 to 1,000 tons.

Building on the experience with the Odd D, large coupled wheels with a diameter of 72 inches and a leading bogie were used again. The Altoona Works delivered 33 double locomotives, the halves of which were never used individually, even if it would have been technically possible. They were designed for a speed of 85 mph. 

The power supply in the tunnels came from a third rail with 650 volts direct current. Each half had a large motor with an hourly output of 2,000 hp, which drove a jackshaft via a connecting rod. This in turn drove two coupled axles. In use, the locomotives proved that they required little maintenance and not only ran steady, but also without making a lot of noise through the connecting rods. With the arrival of the L5 in 1924, some DD1s were initially sold to the Long Island Railroad, which scrapped them between 1949 and 1951. In the PRR, some locomotives were subsequently used to transport empty passenger trains from Penn Station to Sunnyside Yard in Queens. As of 1962 only two of the double locomotives were in service, one of which was moved to the PRR Museum in Strasburg, Pennsylvania in 1978.