Alco's DH643 model, a 4300 hp diesel hyrdaulic

from American-Rails.com

Southern Pacific DH643 #9802 runs light through the engine terminal at Oakland, California, circa 1970. 

The DH643, also known as the "C643-H" or "C643DH," was one of Alco's most powerful locomotive designs, built for Southern Pacific. It resembled UP's C855 design by combining two locomotives into one but used a diesel hydraulic main generator. Both SP and UP experimented with powerful single-unit locomotives to cut costs and had a long-standing relationship with American Locomotive. 

However, the C643DH faced issues similar to the C855, including problems with aluminum wiring that replaced copper, which often caught fire. Southern Pacific retired their fleet of three C643DH units in the 1970s after about a decade of service.

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