Slugs, yard or road, get their traction motor power from a locomotive fitted with the right wiring connections, at low speeds. Above a certain speed, the slugs cut out because the locomotive feeding them can't generate enough power at higher speeds. The goal is to take advantage of the extra power available at low speeds to create more tractive effort, basically turning, for example, a four-axle loco into an eight-axle articulated locomotive. There are yard slugs, with no prime mover and no cab, and are usually low-slung and ballasted to the weight of a regular locomotive. Most are converted switchers with switcher trucks. Road slugs can resemble low-profile yard slugs, or retain their cab (and long hood) as a control cab, and have either their prime mover removed, or in some cases (obsolete or beyond repair) left in place as ballast. They are road engines converted to slugs with their original road trucks. The advantage of a control cab road slug is that it can be used in trains where there are no facilities at the destination where the locos can be turned around (as it was for cab units in A-A, A-B-A etc. sets). Crews love them because ther cabs are very quiet, a relief on longer trips.
In the beginning, there was the Alco-G.E.-I.R. boxcab, often considered the first commercially successful diesel -electric locomotive. The Ingersoll-Rand diesel was primitive, and made 300 horsepower. The twin-diesel, 600 hp unit, if it had better trucks and anyone had ordered one geared that way, had just about enough horsepower to haul one car up to passenger train speeds. The single diesel, 300 horse version barely had enough power to haul itself up to those speeds running light. What they did have was tractive effort. To this day, diesel prime movers do a much better job of making torque than horsepower. Torque and tractive effort are just ways of measuring force. Torque is rotational force. Tractive effort is how much force a locomotive can generate with its steel wheels on steel rail, which is why traction is part of the name. Naturally, the more axles a locomotive has, the more tractive effort it can make, though weight and gearing and other factors weigh in. Horsepower is a measure of how quickly it can keep making more force. You need sheer force to force a stopped train to begin rolling. But you need a lot of force every second for it to accelerate well, and reach high speeds. It needs to make force quickly, and keep it coming. That's horsepower. Early diesels were low on power and high on tractive effort. In the late 1960s the SW1500 had 1500 horses and rode four axles. The F-7A had 1500 horses and rode four axles. By the standards of twenty years later, the shiniest F-7 passenger unit was basically just another switcher. That's how much more power the newest locomotives made. But they didn't ride on more axles. Most locomotives still rode on four or six. Road locomotives didn't need to drag that much weight into motion. They didn't have the horsepower to drag that much tonnage up to speed. But as these more powerful locomotives aged, roads couldn't just retire them to yard duty. They may have liked the idea of using 3,000 horses to shove two hundred cars over the hump at a time, and 3,000 might have been just enough to do it. But these locomotives couldn't get two hundred cars moving by themselves. These semi- retired powerhouses needed more powered axles to do hump yard duties. And it was easier to run fat power cables to a slug than to figure out how to put more axles under the locomotives (and increase their weight too). As for road slugs, locomotives got so powerful that they became better at fast freight and passenger service than drag freight. They had become more like steamers than the early lame diesels, which thrived on drag freight. But one GP-40 and a four axle slug pulled very much like a pair of GP-7s. Either way, it was the same horsepower delivered by the same number of axles, could drag just as heavy a train, and couldn't move it any faster. It was cheaper to buy all high horsepower engines and convert slugs to help them with drag freights than to buy a larger number of both high and modest power engines. Nobody ever said your question had a short, simple explanation...
I agree Kurt but guess it made sense to keep the dynamic braking ability since these were consider road units and not yard only use.
Absolutely! If you need lots of traction to accelerate it, you need just about just as much to decelerate it. Just because dynamic braking can't lock the brakes up doesn't mean all that tonnage on all that grade obeying all that law of gravity can't make the locomotive slide. Just before it leaves the rails and piles up, the locomotive wheels may only be turning fast enough to do 40, but it and the train are still doing 120...
Where's the Milwaukee Road love?! They had some oddballs, especially out here in Washington State, like the "Twinkie", lol!
Nearing 70 years on the rails! Wow! Built as PRR 8679, an RSD-12 built in April 1958, turned into NS MT6 slug in August 2006. A slug, a CRSD20 kitbash job, big trains, cabeese, and an engine terminal to drool over... The slug looks like a homemade job, with that custom-like frame. Definitely Alco trucks. The angle of the photo makes the CRSD20's alligator snout look shorter than it really is.
