The F-Unit is probably the most recognized diesel locomotive in North American history. It was found on railroads from East to West and North to South. Eastern railroads favored a "Single Headlight" version of the F3, while Western railroads favored a "Dual Headlight" version.

F3 locomotives were used in both freight and passenger service. For passenger service, it was necessary for the locomotives to have a Steam Generator to provide heat and electricity in the passenger cars. In later years, many F3 locomotives originally intended for passenger services were downgraded to freight duty.

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