They were ALCO's first diesel switchers to enter true series production, and among the first land vehicles anywhere to use the revolutionary diesel-electric power transmission.

A total of 177 of the HH series was produced; comprising one prototype and four production models of varying power outputs.

The sides of the locomotive's hood sloped outward from top to bottom, and brake equipment was exposed beneath the cab.

The locomotive was equipped with a four-stroke McIntosh & Seymour 531 straight-6 diesel engine, powering a General Electric GT551A1 main generator.

[1] The HH600 was nearly identical to the previous #0900 of the New Haven internally and mechanically, but it was clad in new bodywork, with a straight-sided hood and cab sides that came all the way to the frame.

Birmingham Southern #82, the sole surviving HH900, is on static display at the Heart of Dixie Railroad Museum in Calera, Alabama.