WAM 4 - Father of the Nation for RailFans - Vikas Kumar

The locomotives speed depends on it's traction motor's rated speed ( RPM) , gear ratio and wheel diameter. This is similar to a car or bike's gear system. If you are going up a slope, or carrying something heavy, we usually go in the 1st gear or 2nd gear. But if we have to go fast, we go in the 4th or 5th gear. But a locomotive does not have a fixed gear system. So, either the loco is permanently in 1st gear/2nd gear ( Just an analogy ), when it is useful for freight but can't go at high speeds, or the loco is permanently in 4th gear, where it can't haul heavy loads but can travel very fast. But the point is that you can't have both hauling capacity and speed with the same loco. For example, WAG9 and WAP7 are identical in all aspects ( including the 'aerodynamic'...
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design), but the WAG9 can't go beyond 100-120 kmph because it has a gear box that is designed for freight.
Mixed locos like WDM3D fall somewhere in between. Like there are in the 3rd gear. This can be clearly seen in the ALCOs. WDG3A has a mps of 80-100 kmph, WDM3A at 120kmph and WDP3A at 140-160 kmph. In case of WDM3D, you can see DLW's page and check that the WDM3D has the exact same specs ( Motor RPM, gear ratio and wheel diameter ) as the WDM3A. So, a simple calculation shows that it has the same MPS as the WDM3A and cannot mathematically exceed 120kmph ( except in cases of wheel slip or motor overdrive). Moreover, as the example with the car gear system example explains, if the loco has to run at high speed then it has to have a high gear ratio, which makes it unsuitable for heavy loads. But the WDM3D is designed to handle a bit of both. So it sits somewhere in between the WDG and WDP series, less powerful than the WDG and less faster than the WDP series.