Since both were made by EMD, what is the difference? To my untrained (pun intended) eye, they look the same. One dismal is about as boring as another.
So, what gives?
madwolf
The GP-9 was a 4 axle eng, the SD-9 was a 6 axle eng used a lot in “hump” service.
Yup, like Fred said the SD-9 had 6-wheel trucks. It was built to be used on branchlines and other trackage that was built using lighter weight rail. With it’s 6-wheel trucks it spread it’s weight over a larger area thus allowing it to run on the lighter rail without damaging it.
Peter Harjung said:Hi Peter -
Yup, like Fred said the SD-9 had 6-wheel trucks. It was built to be used on branchlines and other trackage that was built using lighter weight rail. With it's 6-wheel trucks it spread it's weight over a larger area thus allowing it to run on the lighter rail without damaging it.
While I don’t doubt the logic of that statement it makes me wonder why the local branch line has banned all 6 axle locos. They are running older jointed rail that still has places where the roadbed is less than perfect. I thought the ban was due to weight, but perhaps 6 wheel locos put more lateral stress on the rail causing it to spread?
JR
Jon,
According to the "“Model Railroader, Cyclopedia, volume 2"”, EMD built the SD-7 starting in 1952, with a 6-motor design… The SD-7 (1500 HP) had a longer underframe than the GP-7 and was the first EMD locomotive to use the ““Flexicoil”” trucks… The six-wheel felxicoil truck was lightweight, reflecting the SD7’s intended use on branch lines… the design of the truck did sacafice adhesion for the simplicity of maintenence… However, the SD7 also was offered with a heavy duty framefor low-speed drag and transfer operations… They also state that the two additional traction motors were of no significance above 12 MPH, and their sole purpose was to increase the continous tractive effort and to add weighter drivers for greater starting tractive effort…
It goes on to say " The successor, the SD9 (1750 HP), was almost identical, differing externally only in the design of the steps and the classification lights…""
I do believe that over the years, the design feature of the SD's, was to provide more traction power to pull more weight for the longer trains at a greater speed... I also believe, that as most track was up-graded, it was also increased in size, to accommadate the larger and heavier motive power...
I thought for a long time, that the branch line out here, wasn’t capable of handling 6 axle units, because of all the trestles over creeks on the line, however, that was thrown out the window, when a pair of dash-9’s was spotted one day… Prior to that, all I had seen on the line before were GP-38’s and SW-1500’s…
Thanks Andy,
I have it on good authority that this is an actual ban, not simply a matter of the road only owning old 4 axle power. They run GP-9’s and GP-38-2’s. A neighbor road that has track rights runs U-Boats of various models. I’m not real familiar with the GE model numbers.
The closest to any 6-axle power to been seen was a Metro-North FL-9 which is a 5 axle job. 2 In front, and 3 in back. The loco is tail heavy due to a HEP unit to support passenger cars. The photo of the New Haven FL-9 in the woods I was showing at ECLSTS was taking during a special excursion over the branch’s “rare mileage” track.
Jon
The 6 axle SD9’s and other early 6 axle locomotives are an entirely different breed than the 6 axle brutes we have today. As mentioned above, the SD9’s were essentially stretched Geeps with flexicoil trucks. As also mentioned above the SD’s were great at very slow speeds, thus their popularity in yard work. BNSF still uses SD9’s here at Yardley for switching duty.
6100 at Yardley sporting Pumpkin paint and a new cab. You cannot read the writing below the cab numbers, it’s “Radio Control Equipped”. It runs from a TE…
Damn, what a layout! Warren
Jon, anytime…
Warren, yes, it’s a wonderful layout, but I sure wouldn’t want to have to pay for the expense of the upkeep…
Andy Clarke said:
Jon, anytime....Warren, yes, it’s a wonderful layout, but I sure wouldn’t want to have to pay for the expense of the upkeep…
Might not be so bad. At least on that layout you can charge the customers real money. The best I can hope for on my layout is Monopoly money. But then again they are both worth about the same…
Warren Mumpower said:Andy Clarke said:
Jon, anytime....Warren, yes, it’s a wonderful layout, but I sure wouldn’t want to have to pay for the expense of the upkeep…
Might not be so bad. At least on that layout you can charge the customers real money. The best I can hope for on my layout is Monopoly money. But then again they are both worth about the same…
Yeah, thanks to the…
Oops, I said I’d be good, didn’t I?
D*mn election season.
LOL
Jon Radder said:
Hi Peter -While I don’t doubt the logic of that statement it makes me wonder why the local branch line has banned all 6 axle locos. They are running older jointed rail that still has places where the roadbed is less than perfect. I thought the ban was due to weight, but perhaps 6 wheel locos put more lateral stress on the rail causing it to spread?
JR
Good question! My information came from the book: “The Great Northern Railway” by Woods. It states: “Purchased for specialized service were 23 SD-7’s and 27 SD-9’s. These units, riding on 6-wheel trucks, could negotiate very light trackage, get down and lug all day if necessary below 10 mph, and never overheat a traction motor. Many were used as hump engines where their low crawling speed was ideal for pushing long cuts of cars up the hump and then kicking them a few at a time down into the yard.”
But then in “Railroad History #143” it list the weight for the GP-9 (other-wise known at the General Purpose) as 246,475 lbs and the weight for the SD-9 (or Special Duty) as 345,960 lbs. Both used a 1750 hp prime mover.
Yes I know, more questions then answers! 
The weight of a locomotive can vary from railroad to railroad. That 's because the different railroads would request a different weight to get the traction needed. The frames were filled with concrete to vary the weight.
Warren
Uh, oh. In a moment someone’s going to ask how heavy a 45 tonner is …
How much does a 44 tonner weigh?
LESS!!!
Peter Harjung said:
[i][/i] But then in "Railroad History #143" it list the weight for the GP-9 (other-wise known at the General Purpose) as 246,475 lbs and the weight for the SD-9 (or Special Duty) as 345,960 lbs. Both used a 1750 hp prime mover.Yes I know, more questions then answers!
Peter -
Makes sense that a unit with 12 axles is going to weigh more than the same unit with 8. But if you do the math, the weight per axle on the SD is 6% less that the GP, using your prototype examples. Your GP comes in at over 15 Tons/axle while the SD is just over 14 Tons/axle. 1 Ton/axle may not seem like much, but if a track weight limit is 15 Tons/axle, the SD could go there while the GP could not.
JR
PS - My 45 Tonners only weigh about 8 lbs each.
Those locomotives were that size for a reason too. Nothing to do with light track or traction. It was a way for railroads to get around the then required 5 man crew. Federal regs required 5 man crews for locomotives over 45 tons.
Warren
Warren Mumpower said:
Those locomotives were that size for a reason too. Nothing to do with light track or traction. It was a way for railroads to get around the then required 5 man crew. Federal regs required 5 man crews for locomotives over 45 tons.Warren
That figures, I suppose. By the way since UP bought SP some years back I have spotted many different roads going through the L.A. area. Not too long ago, I spotted 3 count em 3 what I think were SD9’s, they were hauling freight past Roscoe. Was nice to see them because SP has been getting more scarce in the L.A. area lately. The one thing I do dig is the Mars light on those. Mars lights and snowplows. I suppose if somebody want something close to the SD’s they could take a USA Geep 7/9 and add the 6 wheel trucks, plus a renumbering. It wouldn’t be absolutely prototype but it would certainly be close and most would never know the better.
When I was in engineer school GP stood for General Purpose/SD Special Duty. Better traction on the 6 alxes. We had restrictions on the Conrail Shiremanstown secondary but not for weight but for curvature of track. Some industries rail made Heavyweights look normal on 8ft dia track,
Marc Bergmueller said:Yeah, its funny though. I remember when SP sent 4 cows probably 1500's out to pull a consist from l.a. to ventura. Watching 4 switchers was actually quite hysterical, but they were moving at a pretty good clip with over 60 cars behind them. So I suppose these days anything goes.
When I was in engineer school GP stood for General Purpose/SD Special Duty. Better traction on the 6 alxes. We had restrictions on the Conrail Shiremanstown secondary but not for weight but for curvature of track. Some industries rail made Heavyweights look normal on 8ft dia track,