ALCO HH series
Portland Terminal Company HH600 #1004, photographed at Portland, Maine in 1968.
Type and origin Power type Diesel-electric Builder ALCO Total produced HH600: 79 HH660: 43 HH900: 21 HH1000: 34
Specifications Configuration: ​  • AAR B-B Gauge 4 ft 8+1⁄2 in (1,435 mm) standard gauge Wheel diameter 40 in (1,016 mm) Length 45 ft 2+3⁄4 in (13.79 m) Prime mover Alco HH600/900: McIntosh & Seymour 531; HH660/1000: McIntosh & Seymour 538 Engine type Inline-6 Four stroke diesel Aspiration HH600/660: Naturally aspirated HH900/1000: Turbocharged Displacement 9,572 cu in (156.86 L) Cylinders 6 Cylinder size 12.5 in × 13 in (318 mm × 330 mm)
Performance figures Power output 600 hp (447 kW), 660 hp (492 kW), 900 hp (671 kW), or 1,000 hp (746 kW)

The ALCO HH series was an early set of diesel switcher 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 first land vehicles anywhere to use the revolutionary diesel-electric power transmission.

The "HH" name stood for "High Hood", a name ALCO came eventually to use in an official context, but originally an unofficial name. Model designations such as HH600 are only semi-official. Original ALCO designations were either descriptive or based on the internal order/design number.

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

The first HH series locomotive, ALCO demonstrator #600, was mechanically almost identical to later designs, but the appearance differed. The sides of the locomotive's hood sloped outward from top to bottom, and brake equipment was exposed beneath the cab. It rode on a unique pattern of trucks. After a period of demonstration on a number of railroads, the unit was sold to the New York, New Haven and Hartford Railroad as #0900. This number often was used to describe the locomotive, although the classification on the builder's data card was "404-OE-200". This first unit was built in July 1931.

The locomotive was equipped with a four-stroke McIntosh & Seymour 531 straight-6 diesel engine, powering a General Electric GT551A1 main generator. Four nose-suspended GE-287-D traction motors in the trucks were geared at a ratio of 4.25:1 to the wheels; the motors were cooled by electrically driven traction motor blowers.^[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. The HH600's were powered by 6-cylinder McIntosh & Seymour 531 engines of 600 hp (450 kW). They were built from July 1932 through May 1939; in all, 78 HH600s were constructed. The first-built units had sharp-edged front hood corners, but in 1934 ALCO employed industrial designer Otto Kuhler to clean up the appearance; he curved the corners and recessed the headlight, and all subsequent HH series units were of this style until another restyling in 1938 where the nose was further rounded. Late versions of this locomotive used the 6-cylinder 538 engine.

The HH900 was a 900 hp (670 kW) version of the HH series using a turbocharged version of the McIntosh & 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 they were supplanted by the McIntosh & Seymour 538T-engined HH1000. Several HH900s were built with the 538T engine.

The HH660 started production in ALCO's lineup in October 1938; 43 examples were built until April 1940. It used a naturally aspirated version of the 6-cylinder McIntosh and Seymour 538 engine, producing 660 hp (490 kW). Externally, HH660s were indistinguishable from late HH600s.

The HH1000 replaced the HH900, with the new McIntosh and Seymour 538T engine, turbocharged to produce 1000 hp (750 kW), a 100 hp (75 kW) increase from the previous model. They were produced between May 1939 and December 1940; 34 were built. M&STL D539 was the only HH1000 built with the 531T engine.

A few HH-series switchers still survive in revenue service, and more are in museums. Working HH locomotives include an HH660 at Gopher Scrap in Mankato, Minnesota, one owned by the Western New York Railway Historical Society, and one from the Delaware-Lackawanna Railroad owned by Genesee Valley Transportation Company at Scranton, Pennsylvania.^[2] Another HH660, Northern Pacific Railway #125, is presently under restoration to her as-delivered appearance at the Northwest Railway Museum.

Among the preserved locomotives are an HH600, four HH660s and an HH1000. Birmingham Southern #82, the sole surviving HH900, is on static display at the Heart of Dixie Railroad Museum in Calera, Alabama.

• List of ALCO diesel locomotives

• Data sheets: HH-600 Archived 2006-10-01 at the Wayback Machine, HH-660 Archived 2006-10-01 at the Wayback Machine
• Komaneski, John. ALCO HH600 Roster. Retrieved March 27, 2005. States information originally from Extra 2200 South.
• Komaneski, John. ALCO HH660 Roster. Retrieved March 27, 2005. States information originally from Extra 2200 South.
• Komaneski, John. ALCO HH900 and HH1000 Roster. Retrieved March 27, 2005. States information originally from Extra 2200 South.
• Laundry, Mark. Alco High Hood Models. Retrieved March 29, 2005.
• Pinkepank, Jerry A. (1973). The Second Diesel Spotter's Guide. Kalmbach Publishing Co., Milwaukee, WI. ISBN 0-89024-026-4.
• Steinbrenner, Richard (2003) The American Locomotive Company A Centennial Remembrance. Chapter VI subchapter "ALCO's First Production Diesels".
• Sweetland, David. (2004) Santa Fe's Alco Switcher Fleet. Diesel Era Vol. 15 No. 6, November/December 2004, pp. 10–31. Withers Publishing, Halifax, Pennsylvania.