History
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 the exhaust equipment. 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 sometimes referred to as a safety cab.
The visibility and access advantages mean that the hood unit is overwhelmingly the most popular style of locomotive in North America, as well as many other nations.
Read more about this topic: Hood Unit
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