Cowl unit

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The GE P42 is one of many cowl designs.

A cowl unit is a body style of diesel locomotive. The terminology is a North American one, though similar locomotives exist elsewhere. A cowl unit is one with full-width enclosing bodywork, similar to the cab unit style of earlier locomotives, but unlike the cab unit style, the bodywork is merely a casing and is not load-bearing. All the strength is in the locomotive's frame, beneath the floor, rather than the bridge-truss load bearing carbody of the earlier type.

Most cowl units have been passenger-hauling locomotives. In this service, the cowl unit's full width bodywork and sleek sides match the passenger cars, do not allow unwanted riders, and allow the decorative, advertising paintwork desired by passenger operators. The cowl unit allows the basic structure of the locomotive to be identical to a freight-oriented hood unit type.

The main disadvantage of the cowl unit is low rear visibility from the cab of the locomotive. The EMD SD50F and SD60F were given a Draper Taper (named after a former CN assistant chief of motive power) where a thinner section of body that widens out to full width is placed behind the cab, although the roof remains full-width the length of the locomotive. This improves rear visibility somewhat, but the locomotives still cannot lead a train in reverse like a hood unit can.