During my visit to the Virginia Museum of Transportation (which is located alongside the NS main in Downtown Roanoke, VA) the power set pictured below came into sight. I don’t see many high-nose locomotives up my way, and that’s what originally got my attention.
As more of the train came into view, the two slugs did as well. They stopped before I could get a broadside, so I had to settle for a so-so shot between the columns of the highway overpass.
Slugs, of course, are relatively commonplace, and I can even understand using two. What gets me about these is the apparent dynamic brakes on the slugs.
That’s a lot of extra traction motors for one engine. A slug works by taking the engine power and distributing it to other motors but has no engine of its own. Therefore there is a lot of traction but low power per axle, which is not going to move very fast. But then again the idea of slugs is to move loads a lone engine would have trouble with, not speed.
no… a slug has no prime mover and cannot generat any power of its own… it draws its power from the mother unit which still has a prime mover… basicly a mother slug combo at slow speeds where tractive effort is needed more is like having 1 8 ax. locomotive…insted of 2 4 ax. locomotives… thus a saveings in fuel for the same amout of pulling power… in a slug car body where the prime mover use to be is normaly a very large block of concret for weight…
There is a possibility that these units aren’t so much slugs as they are “dynamic brake sleds”. The object is not to get the extra traction but to gain extra stopping power. Of course, they could serve both purposes.
Can’t say about these particular slugs, but just as a traction motor needs current to develop traction, it also needs current to set up the magnetic field in the windings to act as a generator and develop braking effort.
Given that the slugs look like they were built for the application rather than some cast-off retrofit job, and given they have cooling fans for dynamic brake, a safe assumption is that the prime mover on the locomotive produces current for the dynamic braking mode for itself as well as its slug companions, but the dissipation of the generated current takes place locally from each unit’s traction motors. There is no law of physics that says you can’t wire the traction motors this way (field windings from the mother unit, motor windings connected to the slug dynamic brake resistor grid), and it seems this is what they are doing.
If you need all of those motors for traction, having an equal number of motors for braking seems to be useful, and you need to spread all of that braking heat dissipation in as many resistor grids as you can. Given that NS serves some rugged mountain territory (especially in VA) where they may need to lug some trains at low speed up a hill, this seems like an reasonable application of that many slugs, and a reasonable place to have dynamic braking on slugs.
This is a pretty unusual capture, I do not think that NS has many of these units. I would suspect that they are purpose built to resolve a local issue, as one respondent mentioned the heavy grades in that area; they would certainly make use of the extra breaking capacity. CSX for some time has used the slug concept on road units, theirs have maintained the cabs on the slugs and apparently can be operated from the cab on the slugs. I have noticed on a couple of instances on UP’s trains running through here, some CSX units that utilized a ‘mother’ unit and slug in the mix.
Maybe some of the NS fans who live in that area can shed some more light on this unusual set of engines.
…Engine’s prime mover most likely produces more power than the 4 axles can put to the rail {traction wise}, and the area must have a certain profile to it that the combination in the photo will do the job {and not needing another engine to use up fuel}.
Not to mention {which has been above}, the extra braking power sure must fit right in for the hilly territory.
It is doubtful the single unit can power both slug units. The electrical connections between three units coupled in this manner would be difficult. I can see the rear unit being deadheaded behind the lead two and taking advantage of the MU brake hose connections to control the brakes between stations, though.
We will have to get the proper answer from some one with NS slugs to provide the proper answer. Two slugs behind one powered unit would be something new in the US rail world.
Two slugs is a common hump pusher on NS. The picture is of one of the Roanoke Yard hump sets. The yard is a downhill hump, hence the need for the extra extended range dynamic braking.
Not really–we tried it once at Proviso–had a GP7 with a slug on either end. I don’t think it served any constructive purpose, and it was shortly after that that SD9s showed up on the hump (knocking out all Geeps, switchers, and slugs), in advance of the SD38-2s that are still there.
