ALCO DH643

ALCO C-643DH
Southern Pacific #9152, formerly #9020.
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
Specifications Configuration: ​  • AAR C-C  • UIC C′C′ Gauge 4 ft 8+1⁄2 in (1,435 mm) Length 75 ft 10 in (23.11 m) Loco 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 Loco 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)
Career Operators Southern Pacific Railroad Numbers 9018–9020 (later 9150–9152; final 9800-9802) Locale North America Scrapped 1973

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 (#9018–#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.