Hood unit
A hood unit, in North American railroad terminology, is a body style for diesel and electric locomotives where the body is less than full-width for most of its length and walkways are on the outside. In contrast, a cab 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 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. 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 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." Starting in the mid to late 1950s, 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" or Canadian 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. Operation. Although the crew cabin is centered on some hood units (particularly in the case of dual, relatively small prime-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 into "long hood" and "short hood" sections. It is generally preferred to run a hood unit "short hood forward" so that the cab is closer to the front, but there is enough visibility in the other direction that they can run "long hood forward" at regular speeds. Some railroad companies (notably, the Norfolk & Western and the Southern) 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. Other locomotives were set up with dual control stands so that they could operate in both directions, making it unnecessary to turn the locomotive around at the end of a run. Some cabless 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. United  Kingdom. 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 as cab forwards when running in reverse. Withdrawn hood unit types include: France. The term "hood unit" is not used in France but a few locomotives of the hood unit type are or were in service: Turkey. Almost all Turkish locomotives have this design. Cab units recently imported and produced under licence. Czechoslovakia. Locomotives of the hood unit type in Czech are: China. Locomotives of the hood unit type  in China are: