The HH series were the first diesel shunting locomotives that ALCO mass-produced in large numbers. They were also among the first land vehicles with a diesel-electric transmission. “HH” stood for “High Hood” while the number stood for the engine power in hp. The first HH600 demonstrator was sold to New Haven after testing in 1931. In 1934, industrial designer Otto Kuhler revised the shape of the locomotive body, giving it more rounded corners.

HH600 No. 913 of the New Haven on a 1938 ALCO advertising brochure

ALCO

The first variant had a McIntosh & Seymour 531 with six cylinders. It wasn't until 1937 that the HH900 followed with a turbocharged variant of this engine. In 1938 the HH660 followed with the slightly more powerful naturally aspirated engine of the 538 series and from 1939 finally the HH1000 with the turbocharged variant of the engine of the 538 series. A total of 177 locomotives were built, of which the HH600 was the most successful with 79 units due to its long production period. The individual railway companies operated a maximum of twelve of each model.