01-29-2015, 03:09 AM
Just to clarify the GP/SD issue. GP stood for General Purpose and came with 4 wheel trucks. SD stood for Special Duty and came with 6 wheel trucks. A GP unit would cost less than a SD unit as the GP has two fewer traction motors and associated electrical equipment. So who would buy a SD? Heavy haul railroads wanting to maximise tractive effort in tonnage trains sometimes with additional ballast and strangely Granger railroads with light track and restricted bridge limits would specify a SD locomotive with reduced ballast. Go figure!
Interestingly there have been no new orders for GP freight locomotives since Santa Fe's GP 60M order in the 90's, everything has been SD up until the recent BNSF order for AC-C4 equipped locomotives, that is a current standard mainline locomotive with 6 wheel trucks but only 2 AC traction motors per truck. This supposedly gives the locomotive the same tractive effort as a DC equipped SD unit. Personally I am not convinced, but what do I know.
The other thing about GP vs. SD is that until the development of radial trucks SD 6 wheel bogies where know to be harder on the track in curves due to the trucks resistance to following a curve.
As far as GP and SD units in local service today the other thing to note is that the railroad will often de-rate a unit and thus lower its total horsepower by removing the turbocharger unit and associated ductwork to remove a high maintenance item which is not really required to perform the job.
In today's railroad world almost anything goes as far as power goes. You can find locomotives from the first generation diesels through to recent new additions like gen-set and battery powered locomotives being used in local service, it all comes down to the railroad and the tonnages and gradients involved.
Hope this helps.
Mark
Interestingly there have been no new orders for GP freight locomotives since Santa Fe's GP 60M order in the 90's, everything has been SD up until the recent BNSF order for AC-C4 equipped locomotives, that is a current standard mainline locomotive with 6 wheel trucks but only 2 AC traction motors per truck. This supposedly gives the locomotive the same tractive effort as a DC equipped SD unit. Personally I am not convinced, but what do I know.
The other thing about GP vs. SD is that until the development of radial trucks SD 6 wheel bogies where know to be harder on the track in curves due to the trucks resistance to following a curve.
As far as GP and SD units in local service today the other thing to note is that the railroad will often de-rate a unit and thus lower its total horsepower by removing the turbocharger unit and associated ductwork to remove a high maintenance item which is not really required to perform the job.
In today's railroad world almost anything goes as far as power goes. You can find locomotives from the first generation diesels through to recent new additions like gen-set and battery powered locomotives being used in local service, it all comes down to the railroad and the tonnages and gradients involved.
Hope this helps.
Mark
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