Road switcher (a.k.a. hood unit) diesel locomotives generally have one short hood and one long hood, with the cab near, but not quite at, one end of the locomotive. While the long hood contains the diesel prime mover and ancillary equipment, the short hood's contents have been more variable with time.
In early road switcher models such as the EMD GP7, the short hood contained the steam generator for heating the cars in passenger service. Radio equipment has also often been housed in the short hood; also a common resident is a lavatory for crew use.
The short hood's primary function is to set the cab back a little from the front of the unit to provide protection for the crew in collisions such as grade crossing accidents. Modern locomotives often include reinforcements, such as a collision post at the front of the short hood, in order to increase this protection.
There have been three major variations in short hood design over the life of the hood unit type on American railroads, which extends from 1949 until the present day. The first road switchers were provided with high short hoods, which were the full height of the locomotive, and gave visibility only through windows at the sides of the front of the cab, in a manner similar to a steam locomotive. The high short hood was good for crew protection, and gave lots of space for equipment. Visibility was better than a steam locomotive's, but still limited; the engineer could not see across the locomotive's nose and had a large blind area there, being unable to see anything reasonably close on the opposite side of the locomotive.
Soon, the low short hood gained prominence. Like the high short hood design, it was narrower than the locomotive, giving good visibility forwards and down, but it was cut off at a level just below the engineer's windows, with extra front windows installed so that the driver had about a 270 degree field of vision, only cut off by the long hood behind him. Apart from a few holdouts like the Southern Railway, the low short hood soon became popular, and eventually the high short hood option was deleted.
More recently, the full-width short hood has become popular. Often referred to as the 'safety cab', 'comfort cab', or inaccurately the 'wide cab' (the nose, not the cab, is wider', the short hood is now the whole width of the locomotive. It's generally slanted at the sides for better visibility. This style began with passenger locomotives and the Union Pacific's EMD DDA40X Centennials, and then became popular in Canada for safety reasons and was soon commonplace in the United States as well. Nowadays, no new road units are available with any other short hood style. Generally the wide short hood was part of a new, redesigned cab package for increased comfort and safety for the crew - EMD calls theirs the WhisperCab