The HH900 was a 900 hp (670 kW) version of the HH series using a turbocharged version of the McIntosh and Seymour 531 engine. Both turbocharged models (HH900 and HH1000) needed a greater cooling capacity, and this was reflected in the larger bodyside radiator space of both models, which distinguishes them from the lower-powered HH600 and HH660. The 21 HH900 units were produced between March 1937 and January 1939, after which it was replaced by the McIntosh & Seymour 538T-engined HH1000. Several HH900s were built with the 538T engine.
Read more about this topic: ALCO HH Series