I've seen references to slugs and pictures of slugs But I'm still not sure what their function and regular use is. I think they serve as a sort of buffer between the Switcher/Locomotive and the cars in the yards/siding it is working. How close am I Or did I completly fall off the curve. BTW: I enjoy just looking at the pictures of the locomotives, trains, and the occasional beautiful Steamers. I wish it was a law that 30% of all woring locomotyives had to be steamers from the 30s and 40s. Thanks See ya Ron
Answer and a Question A slug is connected to a normal engine and has no prime mover, only traction motors. It receives the power for the traction motors from the attached engine. Without the extra engine, a slug has no power to operate. It provides additional traction, usually for switching. A cow-calf is a pair of normally powered engines but the "Cow" has a cab and the "Calf" does not. They are usually MUed in pairs. Both the cow and the calf have motive power - prime mover but the calf is controlled from another engine like the slug. Here's another type that was just used as a brake sled in the yard. Apparently it didn't work well. So why create such an entity? Maintenance? Fewer crews required?
What Flash said... I still don't know why someone would go to the trouble of creating one. Couldn't one just use an old rattle-trap loco as a slug if it's prime-mover is shot?
Most slugs aren't purpose-built. Seaboard had some made in the 80s, and I believe CN as well. CP in the 40s... But most are rebuilt locomotives. Some - the best-known are CSX's slugs from GP30s and GP35s - retain the full carbody and cab.
You still have to wire it to recieve power from the mother. As for the trouble of making one... Take an SD40... it weighs 400,000pds, and is capable of 87,500pds of TE at a max continuous speed of 9.5 miles per hour. Now add a six-axle slug to it that also weighs 400,000pds... that set is capable of 175,000pds TE at 4.75 miles per hour... yet still uses one prime mover. Add yet another six-axle 200T slug... that single prime-mover set is capable of 262,5000pds of TE at ~3mph... quite effective for low-speed high TE applications, like yard service.
Fuel savings. Cost of maintaining a prime mover eliminated. Good for slogging up grades. Remember when BN had their GE "B" units? I always wondered if they'd convert one to a slug as it aged. But never heard of them doing so. Boxcab E50
I'll try to find the picture of the slug with the cab tomorrow. CSX slug without cab Rocky Mount, NC: NS 1991 Atlanta somewhere:
I've seen the former GP30s on the nearby CSX line sometimes. Check my photo album,you may see them there. Usually have numbers in the 2200's range,I think. *EDIT*
Those look pretty bad. The ones I have seen are very spiffy in the Dark Future scheme. April, 2008, Selma, NC.
It seems that hood B-units aren't more likely to be converted to slugs than A-units. I know some of UP's GP9Bs had this fate, but can't recall any others. With cab units, it was more common. Offhand, I can think of CP, CNW, MILW and KCS converting F3/7/9Bs to slugs, and the latter also had F3A slugs that retained their cabs. Note that each of these roads used their slugs differently. CP used theirs in hump service between GP9s, IIRC. I'm not sure about CNW. MILW used theirs between F7As in helper service. KCS used theirs on slow trains, paired with powered F-units either in a powered A/slug B or slug A/powered B set.
I remembered that they had another slug; notice I was referring specifically to F7B slugs. Can't remember right now what they made the other out of.
The SE series actually saw some limited road (transcon main line) service. They weren't truly used in a helper set. It was more of a shuttle service up Tacoma Hill, quite interesting to watch. You could hear them coming long, long before their ever crawling into sight. SG1 was an ex-ALCo RS2, which had been upgraded to what the RR referenced as an RS2.5. Ex-#463 rebuilt to slug 1970. Boxcab E50
A locomotive of a certain weight can only produce a certain amount of tractive effort before the wheels slip. The maximum tractive effort is equal to the coefficient of friction between the wheels and the rails times the weight of the locomotive. At very low speeds, the alternator can usually produce more electrical power than the traction motors use at the maximum tractive effort. A slug is essentially a diesel/electric locomotive with the diesel engine, alternator, and associated equipment removed. It draws electrical power from a "mother" diesel/electric locomotive to power its traction motors. Weight is usually added to the slug to replace the lost weight of the diesel engine/alternator/etc. If this weight wasn't added back, the slug would not be able to produce the same amount of tractive effort before the wheels slip. A slug only helps if the mother engine has extra electrical power available. This usually only happens at low speeds. The slug will cut out above a certain speed or at a certain speed and tractive effort combination. Advantages of a slug vs. using another regular locomotive: No maintenance/capital cost needed for the diesel engine, radiator, alternator, radiator, etc. You may gain some fuel efficiency at low speed, since you run the diesel engine in a more efficient part of its power band. You may gain some efficiency at high speed since you shut down the slug at higher speeds but you don't run the extra diesel engine. You have one less diesel engine idling consuming fuel when you're parked. Disadvantages of a slug vs. using another regular locomotive: You add another equipment type (the slug) to your roster to maintain and schedule. (Schedule as in having the slug where you need it instead of in another yard.) You add another engine type (the mother) to maintain, since most locomotives don't have the electrical equipment and connections to be a "mother." Slugs and mothers are usually kept together as a unit. You lose operational flexibility and reliability. If the mother unit breaks down, the slug is useless. You have to fix the mother or find another mother unit. You can't swap the mother with a regular locomotive. Above a certain speed, the mother unit doesn't have enough electrical power to reach the maximum tractive effort of both it and the slug. The slug has to be cut out. Your train may have a lower top speed and may slow down more on hills. More wear and tear on the mother unit's diesel engine and alternator. I believe KCS had some old F units that they converted to slugs that could run as the head unit of the train. This had the advantage of giving a quieter working environment for the crew. In some ways, a slug is like adding weight to a locomotive. You generally don't try to lighten the weight of a freight locomotive, because you want the extra weight to give you a larger starting tractive effort. You tend to make the locomotive as heavy as the track will support. Once you put all the necessary equipment on the locomotive, you usually end up lighter than you want, so you add weight to the design by beefing up the frame, other components, or adding ballast. If you think of the mother/slug pair as a single, articulated "locomotive", it gives you more wheels to spread the weight over so you can add more weight. (Somewhat simplified, but gives the general idea.) Wikipedia article:
