The PRR P5 was a 2-C-2 electric locomotive that was intended for mixed service. It was designed when it was found out that the O1 introduced in 1930 with its 2-B-2 wheel arrangement was not powerful enough to compete with the Pacific steam locomotives. The two P5 prototypes were built in 1931 with a boxcab design that had its two cabs at the ends. By 1935, 90 P5a had been built by Altoona, Westinghouse and General Electric. The electric part was jointly developed by Westinghouse and GE and was identical on all locomotives.

Each driving axle was mounted rigidly in the frame and powered by two 625 hp motors, for a total power of 3,750 hp. After a fatal grade crossing accident, the last 28 locomotives were built with a central cab and narrower, higher hoods on both ends. This design had recently been introduced with the GG1 and protected the crews in the event of a collision. These locomotives were officially called “P5a (modified)”, but usually only nicknamed “Modifieds”.

In passenger service, they were only used for a short time until the GG1 was introduced. After this point, they were almost always used in freight traffic despite their top speed of 90 mph (145 km/h). A single P5a was rebuilt in 1937 to the P5b by adding traction motors to the four bogie axles. Each axle had a 375 hp motor, for a total of 5,250 hp. The P5b remained a one-off, but stayed in service alongside the others. They were retired in 1965 and today, the prototype No. 4700 is being preserved in the National Museum of Transportation in St Louis, Missouri.