The EMD SD24 was a 2,400 hp (1,800 kW) six-axle (C-C) diesel-electric locomotive built by General Motors' Electro-Motive Division of La Grange, Illinois between July 1958 and March 1963. A total of 224 units were built for customers in the United States, comprising 179 regular, cab-equipped locomotives and 45 cabless B units. The latter were built solely for the Union Pacific Railroad.
Chicago Burlington & Quincy 504, at the Illinois Railway Museum, July 19, 2009; Note the "F" on the sill under the short high hood, indicating the "front" of the locomotive
A B unit, in railroad terminology, is a locomotive unit which does not have a control cab or crew compartment, and must therefore be operated in tandem with another coupled locomotive with a cab. The terms booster unit and cabless are also used. The concept is largely confined to North America. Elsewhere, locomotives without driving cabs are rare.
"B" unit of the Russian 3TE10MK diesel locomotive with a cab-styled body
Russian OPE1A [ru; uk] industrial electro-diesel locomotive for quarry railways with primary electric locomotive and two diesel-electric B units
B unit converted from an EMD F9AM locomotive. The cabin shape was preserved, but its windows were blanked out and the front door was added.
A four-axle EMD GP38-2 B-unit coupled with six-axle SD40-2 A-unit