Cowl unit

The term cowl unit is of North American origin, although similarly-styled locomotives exist elsewhere.

In this service, the cowl unit's full-width bodywork and smooth sides match the rest of the train's general appearance, as well as facilitate decoration of the locomotive with the livery desired by passenger operators, e.g., Santa Fe’s distinctive “warbonnet” livery.

The EMD SD50F and SD60F, GE C40-8M and BBD HR-616 were given a Draper Taper (named after its creator, William L. Draper, a former Canadian National assistant chief of motive power), in which the body is narrower immediately behind the cab, and gradually widens further aft, although the roof remains full-width the length of the locomotive.

This arrangement improves rearward visibility to some extent, but such locomotives cannot run in reverse as the lead unit because they do not have ditch lights at both ends.

These were Great Britain's first mainline diesel locomotives, coming about a decade after America's first cab units.