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Alco's "DH-643": A 4300 HP, Diesel-Hydraulic Design

Last revised: January 16, 2024

By: Adam Burns

The DH643 (also referred to as the "C643-H" or "C643DH" both terms will be used interchangeably in this article) was one of Alco's most powerful locomotive designs. Built for the Southern Pacific the model was similar to UP's massive C855 design in that it was essentially two locomotives merged into one. 

However, it differed by using a diesel hydraulic main generator.  At the time both SP and UP were experimenting with large, powerful single unit locomotives to reduce operating expenses.

In addition, both railroads had been loyal customers to American Locomotive for decades, dating well back into the steam era.   Unfortunately, the C643DH was plagued with many of the same problems UP experienced with the C855. 

Aluminum wiring, for instance, was one such issue.  It displaced copper in a cost-savings attempt.  However, it was completely inadequate and often caught fire.  Following about a decade of service Southern Pacific retired their fleet of three during the 1970s. 


Southern Pacific DH643 #9802 runs light through the engine terminal at Oakland, California, circa 1970. American-Rails.com collection.


The DH643, as classed by Alco, was produced in the early fall of 1964 as the builder manufactured a set of three for Southern Pacific who, like rival Union Pacific, often tested big horsepower in an attempt to reduce the number of locomotives per train.

The SP took this a step a bit further by testing diesel hydraulic designs, which included those produced by Alco (the railroad also operated twenty-one German-built, Krauss-Maffei ML-4000s, three of which were acquired from the Rio Grande).

According to John Kirkland's book, "The Diesel Builders:  Volume Two, American Locomotive Company And Montreal Locomotive Works," Southern Pacific sought domestic production of hydraulic transmission locomotives in an attempt to have both diesel-electric and diesel hydraulic units with common power plants.

Doing so would not only allow for cost-saving maintenance but enable the engines to be interchangeable with other Alco products SP owned.  Ultimately, only American Locomotive elected to move forward with a hydraulic transmission variant.

The C643DH could produce 4,300 horsepower using two 251C model prime movers rated at 2,150 horsepower each. Unlike the C855 which used four B-B trucks, the DH643 carried two, standard C-C (Trimount) trucks and weighed 378,000 pounds.

Internally, the C643DH was basically two C628s merged together albeit with 12-cylinder prime movers. The model used traction motors (model 752) from General Electric although its main generator was produced by Voith Hydraulic, its notable characteristic.  Additionally, air components came from Westinghouse (brakes and compressor).

The locomotive was one of the longest the company ever produced at nearly 71 feet, 6 inches and was capable of producing 95,000 pounds of tractive effort (starting), on par with some of the largest and most powerful steam locomotives ever built.


Only three C643DHs were ultimately constructed for Southern Pacific, originally numbered #9018-#9020 (the railroad later classed them as #9150–#9152 and finally #9800-#9802).

Unfortunately, the locomotive was not particularly successful. First, the diesel hydraulic transmission performed poorly and due to the fact that it was foreign-made meant parts were expensive and took time to arrive. 

Data Sheet and Specifications

Entered Production9/1964
Years Produced9/1964
Model SpecificationDH-643
Engine251C, V-12 (2)
Horsepower4,300 (2,150 per engine)
Carbody StylingAlco
Length (Between Coupler Pulling Faces)71' 6"
Weight378,000 Lbs
Air Brakes26L (Westinghouse)
Compressor3CDB (Westinghouse)
Dynamic BrakesYes
Fuel Capacity4,000 Gallons
Truck TypeTrimount
Truck Wheelbase12' 6"
Wheel Size40"
Traction MotorsGE 752 (6)
Gear Ratio78:22
Main GeneratorGerman Voith Hydraulic
Auxiliary Generator5GY27A (GE)
Tractive Effort Rating95,000 Lbs
Top Speed77 MPH

Production Roster

Total Built = 3

Owner Road Number(s) Construction Numbers Completion Date
Southern Pacific9018-9020*S-3374-01 thru S-3374-039/1964

* Renumbered 9150-9152 (1966), and finally 9800-9802 (1970).


  • Foster, Gerald. A Field Guide To Trains. New York: Houghton Mifflin, 1996.
  • Kirkland, John F. Diesel Builders, The:  Volume Two, American Locomotive Company And Montreal Locomotive Works. Glendale: Interurban Press, 1989.
  • Pinkepank, Jerry A. Diesel Spotter's Guide.  Milwaukee: Kalmbach Publishing Company, 1967.
  • Solomon, Brian. Alco Locomotives. Minneapolis: Voyageur Press, 2009.


Other issues evolved around a lack of proper research and development, similar to the C855.  As a result, the '643s experienced numerous problems and trouble-shooting issues while in service, particularly in regards with the two prime movers functioning as one (timing issues). 

Because of the locomotives' problems, crews came to loathe operating them, which provided for an incredibly jarring ride. 

Dissatisfied with their performance, SP pulled the group from service after less than ten years and all were retired in September, 1973.  Interestingly, UP had scrapped their fleet of C855s a year earlier in 1972. 


Wes Barris's SteamLocomotive.com is simply the best web resource on the study of steam locomotives. 

It is difficult to truly articulate just how much material can be found at this website. 

It is quite staggering and a must visit!