Hood unit

A hood unit, in North American railroad terminology, is a body style for diesel and electric locomotives. On a hood unit, the body of the locomotive is less than full-width for most of the locomotive's length, with walkways on the outside of the locomotive. In contrast, a cab unit has a full-width carbody for the length of the locomotive. A hood unit has sufficient visibility to be operated in both directions from a single cab. Also, the underframe is the main load-bearing member, allowing the hood to be non-structural and easily opened or even removed for maintenance.

The hood unit evolved from the switcher locomotive. A switcher's long hood is normally low enough that the crew can see over it, and there typically is no short hood. Alco introduced the road switcher concept with the RS-1, which was an enlarged switcher with a short hood ahead of the cab. This was added to provide protection for the crew in case of a collision. The low long hood was retained, though its increased length made visibility over it useless. Later, EMD introduced the GP7, which had a similar layout, though both hoods were as high as the cab roof. The high long hood became standard for virtually all hood unit locomotives thereafter.

The long hood of a locomotive is usually about as tall as the cab roof in order to fit the large prime mover and its related subsystems. Originally the short hood of the locomotive was the same height, which is referred to as a high-nose or, confusingly, high short hood. This was originally done to avoid union conflicts, as the high nose ensured that two crewmen (one on each side of the cab) were required in order to see both sides of the track. After this issue was resolved, the height of the short hood was reduced to increase visibility, creating a low-nose or low short hood locomotive. Some locomotives that were originally built with a high nose were later modified to have a low nose. Lately it has become common to make the short hood not only lower but also full-width, creating a wider nose which is usually referred to as a North American Safety Cab.

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