PRR AA1

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The Pennsylvania Railroad's class AA1 comprised two experimental electric locomotives constructed in 1905 at the start of the PRR's electrification project. They were testbeds for larger locomotives to come. Both were of B-B wheel arrangement in the Association of American Railroads classification scheme; each had two trucks, each with two axles and four wheels.

The first, #10001, used gearless motors attached to the axles. The second, #10002, had two differing trucks. The first used gearless motors, while the other used motors mounted to the truck frames and geared to the wheels.

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. It was selected as the basis for the production model, which became the PRR DD1 twin-unit locomotive.