I watched a Trains video on Geeps and at the end, the narrator said that the SD was basically a GP only bigger.
What’s the real scoop here.
Well…GP’s have 4 wheels per axle, SD’s have 6 wheels per axle. I believe that having more wheels assists is spreading out the weight of the locomotive and also helps with wheel slipage, but I’m not an expert in this area.
Meyblc is correct in that GP’s are 4-axle and SD’s are 6-axle. Moreover, GP is short for “General Purpose,” and SD is short for “Special Duty.” Check this out:
http://www.answers.com/topic/list-of-gm-emd-locomotives
Now as to why some are 4 and some are 6 axles, I’m not entirely sure. I don’t know if there is any significant difference between, say, a GP-9 and an SD-9, other than the additional axles.
I heard somewhere that the six-axle units were better-suited for branch line service, where lighter rail was often used. The intent was to have less weight per wheel pressing on the rail. Another common practice on some 1st-generation SDs was to have a pair of smaller fuel tanks (rather than a single full-sized tank), and leave one of the tanks off to reduce the weight even further.
Very interesting question. I had always assumed (yes donkey of you and me) that the SD’s rode on “C” trucks for the additional horsepower from two more traction motors, one additional on each truck. If it was axle loading alone the “E” would have transfered over to drag freight service with their two diesel/generator set on “A-1-A” trucks. My suspicion is that as the horsepower of the diesel/ generator sets increased there were physical limitations on the size of the traction motors mounted on the axle. As horsepower increased wheel slippage became a problem that could not be addressed by increased axle loading alone. In addition the “C” trucks were geared lower for better lugging capability, the ability to start long heavy trains. However the high amperage draw of DC service lead to the development of the AC traction motors for heavy slow service. As I recall the SD’s were for relegated to heavy drag service and the GP’s of similar horspower ratings were used in fast freight applications where speed not pure lugging power was prefered. Perhaps we will get one of the engineers to answer the question definitively. Will
Well if you saw a GP-9 and an SD-9 next to each other, you wouldn’t have trouble telling which was which. Besides the difference in trucks, the SD-9 was/is longer by a couple of feet (don’t remember exact amount, maybe 3-6 ft??) I don’t understand all the technical aspects of it, but apparently even though the GP-9 and SD-9 had the same horsepower rating, having the six motors instead of four did give the SD-9 more pulling power.
Generally SD’s are/were favored for slow heavy freight - think DM&IR ore trains for example, the Missabe had SD-9’s, SD-18’s, and SD-38’s. I don’t know if the SD’s weren’t able to go as fast as the GP’s, but it wasn’t uncommon to see passenger GP’s in the 50’s-60’s, whereas passenger SD’s were fairly rare…again, DM&IR had two SD-9’s with steam boilers for passenger service, Amtrak leased them at one time for the Twin Cities - Twin Ports Amtrak train.
SDs were “Special Duty” as in “Heavy Haul” for mainline use in heavy traffic.
EMD created the designation to suggest a beefier/stronger loco. They usually have more weight built into the frames to assist their dragging ability (get the power down)… so they are not for branch lines with light track. this is why RR like the CNW stuck with loads of small GPs (Geeps) out in the Prairie lines and why so many GP keep getting rebuilt (since the big makers stopped producing ne 4 axle locos).
GP (often written and spoken as “Geep”) were “General Purpose”. I think that this was intended to distinguish them from EMDs earlier F/freight and E/passenger/express units and Alco’s RS units - see below. EMD’s switchers were (mostly, if not all) SWs. Alco’s name equivalent of GP was RS = Road Switcher. Their SD equivalent was RSD = (IIRC) Road Special Duty (??). Their Switchers were “S”. prefixed. (1, 2, 3 etc). They also had “T” prefixed “Transfer” switchers that were dual rated so that they could work out on running tracks with RS and RSD locos.
Never have figured out Baldwin’s Lima Hamilton’s or Fairbanks Morse’s systems of naming… ??? HELP!?
I think that GE got in first with using the letter U at the start of a designation to indicate “Universal” whether a loco was 4 or 6 axle - or even 8.
Just as SD was a sales gimmick this was from the sales department… in this case they also told you the power by the numbers that followed the U… and then th
The “SD” and “GP” or the “C” and “B” trucks were discribing the company locomotive truck type that were made by either EMD or GE.
The GP (4 axle) and SD (six axle) (or F and E units for first generation diesels) were the truck designation for EMD diesel units, while the C (six axle) and the B (four axle) were the designation for GE diesel units.
Thanks guys. This is clearing up some.
In my area, the Buffalo and Pittsburgh, a division of Genesee & Wyoming, has a couple SD-45’s but I’ve never seen them. They bring 100+ car coal drags through town using 3 Geeps or a combination of Geeps and SW1500’s.
The NS in conjuntion with Conrail, especially around Horseshoe Curve, uses SD-45’s for helpers.
Does GE have a version of the Geep, or do the Geeps pretty much own switching?
Well, from this DVD I was watching, like the others already mentioned, GP stands for ‘General Purpose’ which is desgined for pretty much for doing anything from switching to consisting for freight. And SD is for ‘Special Duty’ which originally was designed for those long ‘unit trains’ for better pulling power distribution with 6 axels.
Rilroad like Santa Fe bought a lot of GP20’s for switching purpose but then in the late 60’s and early 70’s, they also used for huge consists some upto 6 or 8 for those huge freight trains. Interestingly enough, in the 70’s, Santa Fe was also the only ones who had bought some GP30 and GP35 B units for their freight operation. I think they must have loed the idea of multi general purpose idea. And in the late 70’s, when Amtrak took over the passenger operation frm Santa Fe, during the Cheyenne leg, UP would actually assign a extra GP38-2 to help to pull up the grades, it was so interesting to do that and that’s what I will do for my layout as well! Because before I was hesitate to mix loco just like the railroad companies themseles in the 60’s. But in the 70’s, they completely changed their idea because of the onset of these GP loco’s and then it’s everything goes from there on. Anything from mixing different models to even different railroad’s loco’s and even EMD E units were pulling mails and mixing passenger cars too with some routes.
Let me reach in and pull out a red herring…
The passenger covered wagons were NOT equivalent to SD hood units. The former had six wheel trucks with two traction motors, while the latter had six wheel trucks with three traction motors.
The A-1-A truck under the P’s was meant to enhance stability at high speed. The C truck under the SD’s was intended to allow greater torque on one hand, and lighter axle loading than a GP on the other, theoretically making it a better drag and branchline engine. The loose link in that idea was that the longer wheelbase of the three-motor truck was less compatable with really sharp branchline curves. (The Grande learned this the hard way after the Monarch Branch was standard-gauged. SD units would derail on the street corner curves, while the GP’s had no problem.)
Chuck (modeling Central Japan in September, 1964)
Well, talking in HO scale, I was told that the SD’s had better trucks and wheels. I was just talking to a hobby shop guy on this matter and he said the difference is the amount of wheels and the ability to pull without spinning and SD’s were a little better quality engine.
Chip,
There hasn’t been a mass produced Geep since the GP60 of the early 90’s. GE had/has 4 axles as well, starting with the U23B and ending with the 8-40B, around the same time as the GP60. The kings of switching today are the GP38-2s and MP15s. The only 4 axle GE’s still around on class 1’s are the 8-40Bs and a few B36-7s, B30-7s and B23-7s. The 4 axle EMDs are much better, from what I’ve heard.
Today, 6 axle units are the only engines EMD and GE produce for the US, the SD70ACe and SD70M-2 from EMD, and the ES44AC and ES44DC from GE. They hold down nearly all freights (although 4 axles work locals and are not uncommon on road trains) while 4 axles (and a few 6 axles) work the yards. A few companies build small 4 axle switcher made from older EMD 4 axles, like GP9s.
An SD’s 6 axles means 12 wheels as opposed to a GP’s 8 wheels which means you have 50% more metal in contact with the rail to improve starting adhesion, this doesn’t make it 50% more powerful as the horsepower of the engine is still the same. Once you are rolling on the flat the number of axles is largely irrelevant which is why hotshot intermodals were frequently GP powered in the past.
Six axles gives better weight distribution too which is why they are good for lighter laid track although you can ballast the loco to put more power down as the wheels will ‘bite’ the rail better. CSX are still adding ballast for locos assigned to coal drags to get the most out of them, see the recent article in Trains.
Individual Railroads will choose 4 or 6 axles for different conditions, gradients, loadings, speed, prevalent weather and years of experience in an area are all factors that they consider. The present trend is for high horsepower all 6 axle locos which seems to be because 4 axles would be pointless with such high horsepower as you’d be spinning all the time.
Doesn’t EMD build a GP20D? I don’t know how many they’ve built or plan to build…But it is listed on the website.
Scott,
As my son would say…Cooool…Niiiiice.
Ok, ok, let’s try to iron out some of the wrinkles, here.
One has to realize that these are old designations, and like typical EMD practice, the meanings change…repeatedly…over the years. What was once true isn’t any longer.
For example, the original concept behind SD’s was for greater weight distribution over lightweight trackage without sacrificing tonnage capacity. Adding axles to any loco will reduce the axle loading (making things easier on track and bridges). But if they are unpowered axles, you lose drawbar tonnage. EMD decided to go with adding only powered axles so as to keep the drawbar tonnage while at the same time reducing the axle loading. Alco had gone both routes with their RSC and RSD series locos, as RSC’s had an unpowered center axle (A1A-A1A), while the RSD’s were all powered (C-C). Early SD’s therefore became the loco of choice for lightweight branchline service for EMD customers.
EMD discovered, however, that splitting the prime mover horsepower over 6 track motors instead of 4 allowed for more traction at low speeds. Why? Because at 12mph or less (or so), the prime mover produces more power than 4 traction motors can use without melting. By adding two more motors, that extra power is put to good use at these low speeds, giving one more tractive effort. This is the same logic behind slugs, as these are simply extra traction motors to be used at low speeds. Therefore, SD’s became the EMD loco of choice for heavy drag freights on mainline service.
Now, to counter that, above 12mph, the extra traction motor really didn’t do anything for you. In fact, it was worse as it made the loco heavier (like carrying around a slug with you all the time), and in “high speed” service, power-to-weight ratio is very important. Therefore, 4-axle locos were marketed to customers who wanted “high speed” service.
Flash forward a few ye
Thanks Paul,
I’ve learned a lot from you and others.
But when look back, my quest for knowledge here was motivated by trying to determine the best switchers for my 2007 Norfolk Southern yard, presumably operated by Conrail. What would I be looking for? I was thinking my Conrail GP-20, would be okay. A Conrail GP-38 would be better–but now I’m thinking about the GP-20D. And to move products through the streets anything from a GP-9 to a SW1500 to MP15.
What would you do?
If I have obtained the correct information through what little is available on the GP20D…The only ones at the moment are owned by CEFX leasing company…So if you are modeling a 2007 yard, it may or may not be appropriate for NS; who knows what they are going to buy next year?
Amtrak has a GP15D…Which looks exactly the same, just without the brake bulge on the hood.
You’ll have to kitbash or scratchbuild a GP20D…I guess you could use a GP9 and lower both hoods and modify the cab.
Yes, I know about the GP20D and GP15D. Again, that’s why I said “mass-produced Geep”. I don’t believe EMD has built any in awhile and probably won’t thanks to MPI’s Green Goat (which looks a lot like a GP20D). Also Chip, get ready to scratchbuild/kitbash. I’m not sure if there’s a GP20D even in brass.
And for your 2007 layout, a GP38 would be fine. It’s close enough to the now more common on NS GP38-2, as the GP20s were retired long ago.