The Pennsylvania built a total of 66 locomotives in its Altoona Works; they operated in semi-permanently coupled pairs.

3936 and 3937, is preserved at the Railroad Museum of Pennsylvania and is listed on the National Register of Historic Places.

Each semi-permanently coupled pair had a length of 64 feet 11 inches (19.8 m) and weighed 313,000 pounds (142,000 kg).

[2] Each locomotive had its own Westinghouse 315-A, direct current, commutating pole, electric motors within a monocoque cab.

[1] Despite their ungainly appearance, DD1s ran "quietly and smoothly...with no appreciable rod clanking", and had a very low maintenance cost.

A total of 66 locomotives were constructed by the Pennsylvania Railroad's Juniata Shops in Altoona starting in 1909; 3936 and 3937 were put into service in 1911.