 Texas Zepher wrote: |
| J Campbell wrote: | |
You would have to research EMDs marketing department records to determine this for certain, but consider this. An SD unit is higher horsepower but has 6-axle trucks so it could be used in special situations where HP is needed but only one unit is to be assigned. Why assign two costly units when one only slightly more expensive unit will do. The downside of the special duty is that it is special in that it cannot necessarily go onto all trackage where a GP unit could. A classic case is the Monarch branch of the D&RGW. A single SD unit would have been ideal for the service but the 18 degree curves were too tight for an SD so they had to use two GPs instead.I don't believe EMD ever imagined that the SD would become the flagship of the fleet when it was first introduced. |
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Another reason was that (early) SD's had lower axle weights than their GP counterparts, so they could run on light branchlines and bridges that the GP's would overload (by axle loadings). That was one of EMD's primary selling points for the early SD's.
Texas Zepher has the right idea, even if he didn't explain it perfectly. The early SD's and GP's (of the same series, say comparing a GP9 and an SD9), had the same horsepower ratings, but since the SD's had 6 traction motors instead of a GP's 4, it had a higher tonnage rating. The GP's were better suited for higher speed gearing and fast running, where the SD's were better suited for lower gearing and heavy pulling.
Today, you don't see 6,000 horsepower GP locomotives, because all that power to 4 axles would do nothing but spin. There are limits on how much power you can transmit to the rails before spinning your wheels (it gets into friction coefficients, adhesion, and other highly technical things). That's one reason why modern (high horsepower) locomotives have 6 axles.
Brad