In North American railroad terminology, a cab unit is a railroad

locomotive A locomotive is a rail transport, rail vehicle that provides the motive power for a train. Traditionally, locomotives pulled trains from the front. However, Push–pull train, push–pull operation has become common, and in the pursuit for ...

with its own cab and controls. "Carbody unit" is a related term, which may be either a cabless booster unit controlled from a linked cab unit, or a cab unit that contains its own controls.

Characteristics

With both body styles, a bridge-truss

design A design is the concept or proposal for an object, process, or system. The word ''design'' refers to something that is or has been intentionally created by a thinking agent, and is sometimes used to refer to the inherent nature of something ...

framework is used to make the body a structural element of the locomotive. The body extends the full width and length of the locomotive. The service walkways are inside the body. Carbody units, gaining rigidity from the body trusswork, require less structural weight to achieve rigidity than do locomotives with non-structural bodies. For that reason, carbody construction was favored to increase the

power-to-weight ratio Power-to-weight ratio (PWR, also called specific power, or power-to-mass ratio) is a calculation commonly applied to engines and mobile power sources to enable the comparison of one unit or design to another. Power-to-weight ratio is a measurement ...

for early diesel locomotives, before the power available with diesel technology was increased. Recent years have seen carbody construction revived in the quest for greater

fuel efficiency Fuel efficiency (or fuel economy) is a form of thermal efficiency, meaning the ratio of effort to result of a process that converts chemical energy, chemical potential energy contained in a carrier (fuel) into kinetic energy or Mechanical work, w ...

with passenger locomotives. The full-width body gives a carbody cab unit poor rear visibility compared with a

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 ca ...

. For that reason, cab or carbody units are mostly used in situations where rear visibility is not important, such as power for through freight and

passenger train A passenger train is a train used to transport people along a railroad line, as opposed to a freight train that carries goods. These trains may consist of unpowered passenger railroad cars (also known as coaches or carriages) push-pull train, ...

s. Cab and carbody units are also more aerodynamic than hood units, and pulled many of the

streamliner A streamliner is a vehicle incorporating streamlining in a shape providing reduced air resistance. The term is applied to high-speed railway trainsets of the 1930s to 1950s, and to their successor " bullet trains". Less commonly, the term i ...

trains.

A and B unit

A cab unit is a carbody unit with a driving cab (or crew compartment). Thus, a cab unit is also always an

A unit An A-unit, in railroad terminology, is a diesel locomotive (or more rarely an electric locomotive) equipped with a driving cab and a control system to control other locomotives in a multiple unit, and therefore able to be the lead unit in a c ...

(a locomotive with a cab). By contrast, a carbody unit can be either an A unit, or a B unit (a locomotive without a cab).

Passenger-oriented cab units

* EMC TA * EMC EA/EB * EMC E1 * EMC E2 * EMC E3 * EMC E4 * EMD E5 * EMD E6 *

EMD E7 The E7 was a , A1A-A1A passenger train locomotive built by General Motors' Electro-Motive Division of La Grange, Illinois. 428 cab versions, or E7As, were built from February 1945 to April 1949; 82 booster E7Bs were built from March 1945 t ...

EMD E8 The EMD E8 is a , A1A-A1A passenger-train locomotive built by General Motors' Electro-Motive Division (EMD) of La Grange, Illinois. A total of 450 cab versions, or E8As, were built from January 1950 to January 1954, 447 for the U.S. and 3 fo ...

EMD E9 The E9 is a , A1A-A1A passenger train-hauling diesel locomotive built by General Motors' Electro-Motive Division of La Grange, Illinois, between April 1954 and January 1964. 100 cab-equipped A units were produced and 44 cabless booster B unit ...

EMD FP7 The EMD FP7 is a , B-B dual-service passenger and freight-hauling diesel locomotive produced between June 1949 and December 1953 by General Motors' Electro-Motive Division and General Motors Diesel. Final assembly was at GM-EMD's La Grange, ...

* EMD FP9 * EMD FL9 * EMC AA * EMC AB6 * ALCO DL-103b * ALCO DL-105 * ALCO DL-107 * ALCO DL-108 * ALCO DL-109 * ALCO DL-110 * ALCO DL-202 * ALCO DL-203 * ALCO FPA/FPB-1 * ALCO FPA/FPB-2 * ALCO PA/PB-1 * ALCO PA/PB-2 * Baldwin 4-8+8-4-750/8-DE * Baldwin DR-12-8-1500/2 ¨Centipede¨ * Baldwin DR-6-4-2000 * Baldwin DR-6-4-1500 * Baldwin DR-6-2-1000 * Baldwin DR-4-4-15 * Baldwin RF-16 * Baldwin RP-210 * Fairbanks-Morse Erie-Built * Fairbanks-Morse CPA-20-5 * Fairbanks-Morse CPA-24-5

Freight-oriented cab units

* EMD FT * EMD F2 * EMD F3 * EMD F7 *

EMD F9 The EMD F9 is a Diesel-electric locomotive produced between February 1953 and May 1960 by the Electro-Motive Division of General Motors (EMD) and General Motors Diesel (GMD). It succeeded the F7 model in GM-EMD's F-unit sequence. Final asse ...

* ALCO FA/FB-1 * ALCO FA/FB-2 * Fairbanks-Morse CFA/B-16-4 * Fairbanks-Morse CFA/B-20-4 * Ingalls 4-S

Cowl unit

cowl unit A cowl is an item of clothing consisting of a long, hooded garment with wide sleeves, often worn by monks. It was developed during the Early Middle Ages. The term may have originally referred to the hooded portion of a cloak, though contemporar ...

is an adaptation of the

design with a full-width body. Despite some visual similarities, cowl units are actually very different from cab units. All structural support on a cowl unit is provided by the frame of the locomotive, rather than in the body as with a cab unit. This allows manufacturers to cheaply and easily create full-width locomotives from their hood unit designs by simply adding cowling. Cowl units were first introduced as a special order from the Santa Fe, which wanted a sleeker design for its passenger equipped hood units. Although the first cowl units (such as the EMD FP45 and the GE U30CG) were meant for passenger service, EMD would later offer freight-only derivatives starting with the F45.

Great Britain

Cab units were not generally used in Great Britain. The traditional makers continued to use heavyweight frames and

s instead. The LMS twins 10000 and 10001 used the design and later locomotive types such as the British Rail Class 37, and

British Rail Class 40 The British Rail Class 40 is a type of British railway diesel electric locomotive. A total of 200 were built by English Electric between 1958 and 1962. They were numbered D200-D399. Despite their initial success, by the time the last examples w ...

utilised cab units but the term "cab unit" is not used in Britain. The Class 37 and Class 40, like most British diesel and electric locomotives, has a cab at each end.

References

{{Diesel-loco-stub Diesel locomotives