A cabless diesel locomotive (meaning that it isn't manually powered by a crew or driver)

A cabless diesel locomotive; or a "B-unit". With it being a former BN SD40-2B (rebuilt from a SD40-2 cab unit).

is a cabless form of diesel locomotive lacking a cab built to serve as a "booster" in a consist (though, sometimes a rebuilt or downgraded locomotive converted from a cabbed unit) which helps provide more horse-power for the leading locomotive, to which the leading locomotive (or another in a consist) is what powers it per MU cables.


There are many different nicknames and forms of an unpowered diesel locomotive. Such as: the "B-unit", "slug", "slave", or "booster" and several other terms (though, slugs are a common form of power units similar to "B" units). Many American railroads and British railways are the main users for these particular forms of power, even though some can be found throughout other parts of Europe and North America.

They were first officially built by EMD for their "F-series" and "E-series" lines of locomotives during the 1930's to the 1960's.


There are several versions of a cabless diesel locomotive:

"Road Slug"- A cabless unit (usually 4 axle) to assist horsepower on an OTR consist.

"Yard Slug"- A converted or rebuilt locomotive with fuel lines supplied from the cab unit. (Usually cabbed.)

"B-Unit"- A cabless diesel locomotive from the originating F series locomotives. (The "B" is often known or refers to the term "booster".)

"Slave" - A cabless unit with remote control; another term for a "slug".

"CCRCL" - A remote controlled downgraded yard slug. (Usually cabbed.)


Because of GE's diesel locomotive designation being from series, to horsepower rating, to axle load (such as the B23-7 and C44-9W; both having the distinct names) their designation for "B-units" was never added. Railfans often call the BN B30-7A units B30-7ABs or B30-7A(B)s. However GE and BN did not add the additional B at the end.

Norfolk Southern recently aqquired remote controlled slugs similar to B-units to be compatable with their RailPower and Genset switchers rebuilt from older diesel locomotives.