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SD = Special Duty <br />GP = General Purpose <br /> <br />SD units were built for use on lines with lower weight limits - they spread the weight of the loco over more axles so the weight per axle is lower. More recently, they have allowed more tractive effort to be exerted. GP's were intended for normal use in pretty much any job, though to the best of my knowledge there have been no new GP units since the GP60M and GP60Bs built for the Santa Fe's fast intermodals - I'm guessing the SD's now offer the best mix of power, weight, fuel capacity, etc.
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