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A road switcher is a type of railroad locomotive which was originally employed for delivering or picking up railcars outside of a railroad yard. Both type and term are North American in origin, although similar types have been used elsewhere. Since the road switcher must work some distance away from a yard, it must be able to operate at road speeds, it must also have high-visibility while it is switching, and it must have the ability to run in both directions. Additionally, a road switcher must have the power rating and cooling capacity of a traditional road engine, and a road switcher must have high-speed (i.e., road) trucks, not low-speed (i.e., switcher) trucks. Modern road trucks are always equipped with (frictionless) roller bearings; switcher trucks were almost always equipped with (friction) plain bearings, until such bearings were outlawed in interchange service on railcars and locomotives.
For the reasons given above, road switchers are generally hood units. The set-back cab of a hood unit provides more safety in the event of a collision at speed than most switcher designs, and the rear visibility is much better than that of a cab unit. Due to their ability to both run at road speeds for long distances and to switch cars, road switchers, as their name implies, are often used for road (heavy-haul) duties, in addition to their yard (switching) duties. Since the 1960s, road switchers have completely displaced cab units in heavy-haul freight service (but cab-type units, adapted from certain road switcher prototypes, have been employed for contemporary passenger service, in selected cases). Some road switchers were provided with twin control stands, so that the units could operate conventionally (locomotive engineer and conductor/switchman facing the direction of travel) in either "long hood forward" or "short hood forward" directions. However, twin control engineer positions have fallen into disuse as almost all operations are now run "short hood forward". For obvious reasons, the short hood is labeled "F" (meaning "front").
Alco's RS-1 was the first successful example of the type, and virtually all modern hood units are laid out in a similar fashion (long hood for all propulsion equipment, short hood for crew accommodations including a toilet). The RS-1, being the first example of a road switcher, and having been initially developed when plain bearings were still common (although not on cab-type road units), often were equipped with plain bearings. Subsequently, roller bearing conversions were implemented, and new units were generally ordered with roller bearings. The RS-1 had a very long manufacturing history, so most 1940s units might be initially ordered with plain bearings (and subsequently converted to roller bearings), but most 1960s units might be ordered with roller bearings.
Fairbanks Morse entered the road switcher field in 1947 with the H-20-44.
Although Alco produced the first known road switcher, EMD's GP7 was probably the most successful model from this early period road switchers. Few or no EMD GPs and no EMD SDs were ordered with plain bearings, and any plain bearing-equipped GPs were later updated to incorporate roller bearings.
Although it is always controversial to generalize about "generations" of road switchers, these ubiquitous beasts of burden may be divided into: Generation 1, 1,999 horsepower (1,491 kW) or lower, net for traction; Generation 2, 2,000 horsepower (1,500 kW) to 2,999 horsepower (2,236 kW), net for traction; Generation 3, 3,000 horsepower (2,200 kW) to 3,999 horsepower (2,982 kW), net for traction; and Generation 4, 4,000 horsepower (3,000 kW) or higher, net for traction. Although at one point 6,000 horsepower (4,500 kW), net for traction, units were made, these quickly fell into disuse, and most have been scrapped by North American railroads. The most common new units made today are 4,300 horsepower (3,200 kW) to 4,500 horsepower (3,400 kW), net for traction.
Within the Americas, road switchers are almost always diesel-electric, with the "transmission" system (i.e., the final drive) being either direct current (standard performance units) or alternating current (high performance units). For economic and performance reasons, 2,500 horsepower (1,900 kW) and lower units generally had a dc generator, producing 600 volts dc, nominal, whereas 3,000 horsepower (2,200 kW) and higher units generally had an ac alternator with integral rectifier, producing 1,200 volts dc, nominal, (alternator/rectifiers remained an option on certain sub-3,000 horsepower (2,200 kW) units, for economic and service reasons). Units with ac final drive accepted the 1,200 volts dc from the alternator/rectifier and inverted this to 1,200 volts three-phase variable frequency ac.
PKP class SM42 is a Polish 74-ton diesel locomotive used for shunting, light main railroad cargo haulage, and passenger service (version SP42 and SU42). 1822 units were built, used mostly by Polish carriers but some were exported abroad.
- BL, meaning "branch line"
- Identification of this generation, as well as others, is occasionally blurred by the introduction of supplementary models; For example, EMD's Generation 3 40 Series models initially included the 3,000 horsepower (2,200 kW) GP40 and SD40 and the 3,600 horsepower (2,700 kW) SD45, however this series was later supplemented by the 2,000 horsepower (1,500 kW) GP38 and SD38 and the 2,300 horsepower (1,700 kW) GP39 and SD39, all of which were constructed using the same major components (frame, carbody, cab, etcetera), and the 38 Series eventually became one of EMD's best sellers; Indeed, many of these early units were later upgraded to incorporate Dash 2 subsystems, for improved functionality and reliability; the later 50 Series (3,500 horsepower (2,600 kW)) logically belongs to this generation while the still later 60 Series (3,800 horsepower (2,800 kW)) logically belongs to Generation 4.
- Generation 1 and 2 units incorporated conventional (discrete) locomotive controls; Generation 3 units generally incorporated modular (plug-in) locomotive controls; Generation 4 units generally incorporated microprocessor-based locomotive controls.