Southern Pacific Alco DH643 Diesel Hydraulic No. 9152 (formerly No. 9020). Click to enlarge.

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The ALCO C-643DH, also known as the Century 643DH, was a twin-engine diesel-hydraulic locomotive, the first diesel-hydraulic road switcher built in the United States. It had a C-C wheel arrangement and generated 4,300 horsepower (3,200 kW). Only three were built, all for Southern Pacific Railroad in 1964 (No. 9018–No. 9020). The Alco C-643DHs joined 21 Krauss-Maffei ML-4000 diesel-hydraulics already on the Southern Pacific's roster. They spent most of their service lives in the flat San Joaquin Valley in California.

Dissatisfaction over the poor performance of diesel-hydraulic locomotives, as well as their use of foreign-made components (the hydraulic transmission was of German Voith design), eventually led Southern Pacific to sell the 3 C-643DHs for scrap in 1973. None of the 3 examples built survived into preservation.



Type and origin
Power type: Diesel-hydraulic
Builder: American Locomotive Company (ALCO)
Order number: S-3374
Serial number: S-3374-01 – S-3374-03
Model: DH-643
Build date: September 1964
Total produced: 3
​• AAR C-C
• UIC C′C′
Gauge: 4 ft 8+1⁄2 in (1,435 mm)
Length: 75 ft 10 in (23.11 m)
Locomotive weight: 378,000 lb (171,000 kg)
Prime mover: ALCO 251C, (x 2)
Engine type: V12 Four-stroke diesel, (x 2)
Aspiration: Turbocharger
Displacement: 8,016 cu in (131.36 L)
Cylinders: 12 (x 2)
Cylinder size: 9 by 10+1⁄2 inches (229 mm × 267 mm)
Transmission: Voith hydraulic
Locomotive brake: Straight air
Train brakes: Air
Performance figures
Maximum speed: 77 mph (124 km/h)
Power output: 4,300 hp (3,200 kW)
Tractive effort: 95,000 lbf (422.58 kN)
Operator: Southern Pacific Railroad
Road Numbers: 9018–9020 (later 9150–9152; final 9800-9802)
Locale: North America
Disposition: All scrapped 1973