High Quality Content by WIKIPEDIA articles! The ALCO HH series were an early series of switcher diesel-electric locomotives built by the American Locomotive Company (ALCO) of Schenectady, New York between 1931 and 1940, when they were replaced by the S series; the 660 hp (490 kW) S-1 and 1,000 hp (750 kW) S-2. They were ALCO`s first diesel switchers to enter true series production, and among the very first land vehicles anywhere to utilise the revolutionary diesel-electric power transmission.