A cab unit and a carbody unit, while closely related, are not exactly the same thing. Both are examples of railroad terminology. Both are body styles of locomotives. With both body styles, a bridge-truss design framework is used to make the body a structural element of the locomotive. The body rises above the locomotive frame, and extends the full width of the locomotive and along its length. The service walkways are inside of the body. This gives a cab unit poor rear visibility compared to a hood unit. For this reason, cab units are mostly used in situations where rear visibility is not important, such as power for through freight and passenger trains. Cab units are also more aerodynamic than hood units, and pulled many of the streamliner trains.

Cab units are carbody A units. In other words, a "cab unit" refers only to A units, while a "carbody unit" refers to both A units and B units. Therefore, a cab unit has to have a driving cab, or crew compartment. A carbody unit does not.

Lately, locomotive manufacturers have switched from cab units to cowl units when a full-width body is desired. Since all the structural support on a cowl unit is in the frame of the locomotive, rather than the body, manufacturers can easily create full-width locomotives off of hood unit designs if they build cowl units.