North American Rails - Railfan Guides
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.
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North American Rails - Railfan Guides