The Pennsylvania Railroad's class DD1 were semi-permanently coupled pairs of third rail direct current electric locomotives built for the railroad's initial New York-area electrification. They operated between Manhattan Transfer and Pennsylvania Station in New York City, and from there to the coach yards at Sunnyside Yard in Queens, New York. Some also operated on the PRR-owned Long Island Rail Road's extensive third rail network. They had a wheel arrangement of 4-4-0+0-4-4 in the Whyte notation; in other words, two 4-4-0 locomotives coupled back-to-back. In the AAR wheel arrangement notation, this is described as 2B+B2. Each half-locomotive contained one large electric motor mounted in the body and driving via a jackshaft and side rods.
- Staufer, Alvin (1962). Pennsy Power. Staufer, 248–253. LOC 62-20878.