The first, No. 10001, used gearless motors attached to the axles. The second, No. 10002, was built with motors mounted in the truck frames and geared to the wheels. Later on it was given one gearless truck.
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 DD1twin-unit locomotive.
At low speeds in switching service, however, the locomotives were acceptable. The first, #10001, was renumbered #3950 and was sold to the (PRR subsidiary) Long Island Rail Road in 1916. Numbered 323 on that road and nicknamed "Phoebe", the locomotive was in use until 1937 when it was scrapped. No. 10002, meanwhile, was renumbered #3951 and continued in service on the PRR.