The Pennsylvania Railroad's class AA1 comprised two experimental electric locomotives constructed in 1905 by the company's own Altoona Works with the assistance of Westinghouse.

Intended as testbeds as the PRR began its electrification project, both locomotives remained service into the 1930s.

In the early 1900s, the Pennsylvania Railroad embarked on a major program to improve its access to New York City.

[6] Both locomotives proved unstable at speed, pounding the track with high lateral forces.

A competing experimental unit, "Odd D" #10003, of 4-4-0 wheel arrangement in Whyte notation or 2-B in the AAR scheme, proved much more stable.