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Ahood unit, inNorth Americanrailroad terminology, is a body style for diesel and electriclocomotives where the body is less than full-width for most of its length and walkways are on the outside. In contrast, acab unit has a full-width carbody for the length of the locomotive and walkways inside. A hood unit has sufficient visibility to be operated in both directions from a singlecab. Also, thelocomotive frame 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 theswitcher 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 theroad switcher concept with theRS-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 theGP7, 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 largeprime mover and its related subsystems. Originally the short hood of the locomotive was the same height, which is referred to as ahigh-nose orhigh short hood. Starting in the mid to late 1950s, the height of the short hood was reduced to increase visibility, creating alow-nose orlow 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 aNorth American Safety Cab orCanadian comfort 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 regions.
Although the crew cabin is centered on some hood units (particularly in the case of dual, relatively smallprime movers), in most cases the cab is closer to one end of the locomotive than the other (in the case of a single, relatively large prime mover), breaking the locomotive up intolong hood andshort hood sections. It is generally preferred to run a hood unitshort hood forward so that the cab is closer to the front, but there is enough visibility in the other direction that they can runlong hood forward at regular speeds. Some railroad companies (notably, theNorfolk & Western and theSouthern) ordered locomotives with cabs facing long hood forward so that the short hood is actually the rear of the locomotives, but that practice has become increasingly rare. This was usually done to offer greater protection to the crew in the event of a collision and to give better visibility of trackside signs and objects. Other locomotives were set up withdual control stands so that they could operate in both directions, making it unnecessary to turn the locomotive around at the end of a run. Somecabless hood units were also built. The long hood ran the whole length of those locomotives. In North America, all locomotives are required to have the letter F printed on the side sill at the end which is normally operated as its front.
The term "hood unit" is not used in the UK but a few locomotives of the hood unit type are in service. Some of these locomotives may also be classified ascab forwards when running in reverse.
Withdrawn hood unit types include:
The term "hood unit" is not used in France but a few locomotives of the hood unit type are or were in service:
The term "hood unit" is not used in Germany but many switcher locomotives and a few freight locomotives of the hood unit type are in service:
Almost all Turkish locomotives have this design. Cab units recently imported and produced under licence.
Locomotives of the hood unit type in Czech are:
Locomotives of the hood unit type in China are: