| ALCO C-643DH | |
| Powertype: | Diesel-hydraulic |
| Builder: | American Locomotive Company (ALCO) |
| Ordernumber: | S-3374 |
| Serialnumber: | S-3374-01 – S-3374-03 |
| Buildmodel: | DH-643 |
| Builddate: | September 1964 |
| Totalproduction: | 3 |
| Aarwheels: | C-C |
| Uicclass: | C′C′ |
| Length: | 75feet |
| Locoweight: | 378000lb |
| Primemover: | Alco 251C, (x 2) |
| Enginetype: | V12 Four-stroke diesel, (x 2) |
| Aspiration: | Turbocharger |
| Displacement: | 8016cuin |
| Cylindercount: | 12 (x 2) |
| Cylindersize: | 9× |
| Transmission: | Voith hydraulic |
| Maxspeed: | 77mph |
| Poweroutput: | 4300abbr=onNaNabbr=on |
| Tractiveeffort: | 95000lbf |
| Locobrakes: | Straight air |
| Trainbrakes: | Air |
| Operator: | Southern Pacific Railroad |
| Fleetnumbers: | 9018–9020 (later 9150–9152; final 9800-9802) |
| Locale: | North America |
| Scrapdate: | 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 4300hp. 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.