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Thread: Uses of GP and SD Locomotives

  1. #1
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    Question Uses of GP and SD Locomotives

    This may have been discussed before but,
    Besides the size, trucks, performance, etc of EMD's GP and SD locomotives, I was wondering what a GP locomotive would be used for in comparison to an SD locomotive. I've heard that GP locomotives are used for road switching sometimes. Are they commonly used in mainline freights? What would make a railroad prefer to use a GP loco over an SD loco, or vice-versa? On top of that, have switchers ever served another purpose besides switching, and what does the F in F45 or F40ph stand for? Answers would be appreciated.
    Thanks,
    Erik
    Last edited by cookiebinks; December 6th, 2011 at 12:13 AM.

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    what does the F in F45 or F40ph stand for
    Let me answer this one first. The EMD F40PH is a passenger version of the EMD GP40. Therefor the letters stand for Full Body, 40 series, Passenger, Hood unit.

    as for a road preference to the use of GP (general purpose) over SD (special Duty)(trade terms) locomotives it all comes down to weight of the train and availability of road power on hand. I know some would want to make it more complicated but it is really that simple. Hope this helps.

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    Default Way back when...

    Back in the beginning of Dieselization in the 1940's railroads replaced steam engines in kind, one Diesel replaced one steam engine. The EMC FT at 1350-hp was the first mass produced locomotive, a four axle "covered wagon" carbody in 1939.

    The American Locomotive Company (ALCo) developed the RS-Series "road-switcher," a departure from their carbody series FA-units.

    Dick Dilworth purposely developed the GP-7 locomotive as then EMD's answer to the RS3, as "ugly" as possible so that railroads would keep it out of site on branchlines or whatever and out of sight of the public...

    Depending on whether their was competition as in most of the country, or in the poorer Southeastern US-America, railroad's track maintenance ultimately determined the use of the new six-axle locomotives that came out in the early 1950's.

    Some railroads had tight turnout (switch) radius an the ALCo RSC-2 and RSC-3 was made with an unpowered center axle between two motors. This was also to reduce the weight per axle on the rails, however it also lifted the driving axles and allowed wheel slip.

    Yes, six axle powered locomotives made for heavy tonnage low speed were preferred by the larger roads, also some ran fast freights with these but they cost more and when you send a unit for unit to trade, you are out of so many units and you can build a four-axle unit faster than a six-axle unit at lower cost.

    Again, jointed rail predominated the South in areas like all of Florida south of Jacksonville to Orlando, etc. Even though FEC had welded rail on concrete ties, four axle locomotives ruled the entire line from Jax to Hialeah.

    Today, all the Class One's run welded rail, wide cabs, high-adhesion high horsepower six axle units. The last four axle locomotives produced were the EMD GP60 and GE Dash-840B's

    Now, almost every road runs their older six-axle locomotives for local freights and transfer runs, however the SD40's are "warmed over" occasionally.

    The four-axle units become slugs, slug-mothers or are simply sold to eventually become shortline, regional or lease units.

    Homework is Wikipedia, or a host of fansites for Diesel-electric Locomotives...Google that!

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    You might find this site helpful http://www.alkrug.vcn.com/rrfacts/rrfacts.htm

    It is produced by a veteran BNSF engineer. The major difference between General Purpose vs. Special Duty locomotives has to do with the relationship between horsepower and tractive effort. A bit oversimplified though this may be, basically if you have a line going through difficult terrain, such as the Overland Route through the Sierras you would tend to favor SD locomotives hauling slower trains over the heavy grades and through the sharp curves. On the other hand, roads like the D&RGW or the Santa Fe tended to favor faster, shorter trains, calling for GP locos. His essay on this subject will probably answer most of your questions.

    Bernie
    Last edited by bl4882; December 6th, 2011 at 09:35 AM.

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    Tight radius curves, and "two bit trackage" branchlines, usually never saw anything bigger than a GP's ... SW's and other very small wheelbase locos were the normal motive power.
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    Quote Originally Posted by cascaderailroad View Post
    Tight radius curves, and "two bit trackage" branchlines, usually never saw anything bigger than a GP's ... SW's and other very small wheelbase locos were the normal motive power.
    i wouldn't say that. some SDs were made with the intention they be used on light track on branch lines that cant handle the weight of a locomotive on 4 axles.

    the GP or SD comes down to adhesion, speed, weight per axle, and hp requirements that a particular railroad - or line, or even single train - might have.
    Last edited by norfolksouthern37; December 6th, 2011 at 11:40 AM.

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    Quote Originally Posted by bl4882 View Post
    You might find this site helpful http://www.alkrug.vcn.com/rrfacts/rrfacts.htm

    Bernie
    Thanks for linking that site, Bernie. Lots of interesting articles on there. I learned a lot more about how railroads operate.
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    Like Justin said, it's logical to think that 4 axles would be better for branch lines due to the tighter turning radius, but since the 6 axle spreads the weight out they're actually easier on older and/or lighter weight rails. Better traction too, a steel wheel on steel rail has a footprint about the same surface area as a dime, so with a Geep you get 80 cents worth of traction, an SD gives you a dollar and twenty cents.

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    yea thats one way of looking at it...


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    Quote Originally Posted by sniper297 View Post
    Like Justin said, it's logical to think that 4 axles would be better for branch lines due to the tighter turning radius, but since the 6 axle spreads the weight out they're actually easier on older and/or lighter weight rails. Better traction too, a steel wheel on steel rail has a footprint about the same surface area as a dime, so with a Geep you get 80 cents worth of traction, an SD gives you a dollar and twenty cents.
    The MILW RR had ten SDL39's that were specially made for branch line service due to the poor trackage. The unit was built on a short 55-foot, 2-inch frame with customized export-style trucks, barely tipping the scales at 250,000 pounds and managing a light-footed axle-loading of just 20.8 tons per axle. Wisconsin Central inherited 9, these were all sold to a South American RR.

    John

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    Default I wanted them in the Bone Valley...

    I liked the SDL39, there was fuel economy from the Series 12V645 as well as light weight.
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    Yeah but SD power only gives you snail speed while GP power gives you road runner or cheetah speed plus you could just add extra units plus they're cheaper and depending on the loco it gives the same horsepower as and SD loco just less traction but greater speed and lower costs. \

    It just depends on what you want. Heavy freight would best be served by SD power, where branch-lines with tight curves and hotshot fast freight trains would be best served by GP locos due to greater acceleration and speed. Plus GP locos are more mobile and can stop faster and start faster due to lighter body weight.

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    Not quite correct, GP's advantage over SD is in acceleration, all commuter and Amtrak use 4 wheel units for this reason. The SD units are more than capable of reaching the same speeds as the GP. The speed is in the gearing of the wheels not the wheel types, BB or CC. Also if that were true the railroads would have more 4 axle units on their roster than six axle units, heck, the major builders do not catalog 4 axle freight locomotive at the present. The only 4 axle new units are used for low speed switching or local area operations and most of these are Gensets.

    As far back as the late 70's UP had the SD40-2 regeared so that their top speed was 80mph, the same speed as their DD40X. Except in Amtrack's NEC, where can you find speeds over 100MPH?

    John

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    The CNR and CPR freights around Brighton go at speeds of 155 km/h, and they're Co-Co's
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