In North American railroad terminology, a cab unit is a railroad "locomotive" 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 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 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 with passenger locomotives. The full-width body gives a carbody cab unit poor rear visibility compared with a hood unit. 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 trains. Cab and carbody units are also more aerodynamic than hood units, and pulled many of the streamliner 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 (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 EMD E8 EMD E9 EMD FP7 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 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 A cowl unit is an adaptation of the hood unit design with a full-width body. Despite some visual similarities, cowl units are actually very different from cab units. All structural