37 Photos included after text.

B units hold a fasination with me because they are just a little different.  Without the usual cab for the crew, they have a very destinctive appearence.  Some of the first B units were with the early diesels in the F and E series, and were offered from every locomotive manufacter in the early years.

Once engine designs changed with the introduction of EMD's GP series the idea of a B unit continued.  However, around the 70's the railroads thought that the B units were not that versatile in that they always had to be run with a regular locomotive.  To the railroads way of thinking, they were not that cost effective and for the most part, railroads stopped buying B units.

However, railroads had diesel engines get in wrecks and if it was possible to repair them, they would.  Some railroads, most notably the Santa Fe, rebuilt those locomotives to a B unit rather than rebuild the expensive cab.  Union Pacific took the idea in another way and just took out the controling equipment of some of their locomotives in a cost saving measure, and they also became "B" units even though they still retained their cabs.

Other railroads rebuilt damaged units in a similar fashion to save money and keep a working unit on the roster.  Those railroads had a few examples but not many.

The Santa Fe surprized alot of people with the purchase of the GP60B's to go along with their superfleet locomotives.  

The main difference between a B unit and a slug is that a B unit is pretty much a regular engine without the cab area for the crew.  It still has all the components of a locomotive.  Therefore a B unit still has a prime mover, generator and traction motors and could move without a "mother" unit, however it rarely does.

A slug is just the traction motors for the axles and some added weight for traction.  It cannot run by itself and is completely dependent on the "mother" unit.

I have also included a couple of photos of the fuel tenders that the Burlington Northern used on some of its engines for a variety of reasons.