WDM3D 160kmh pe capable hai but 120kmh pe restricted hai.Bcoz wdm3d
ka design aerodynamic nahi hai,isliye ise 0 se 160kmph tak accelerate karne me bahut time
lagta hai.Phir bhi kabhi kabhi SBC,DBRT Rajdhani ke saath dikh jaata hai.
is problem keliye DLW ne ek special aerodynamic 11121 WDM3D banaaya jo
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GY shed ke paas hai,par yebhi itna succesful nahi hua.Isliye iske baad se EMD locomotives aerodynamic design
waale banne lage for fast acceleration.Elocos bhi saare aerodynamic design
waale bante hai for quicker acceleration like WAP5/7.yahi problem ki wajah se WDM3D ko
120kmph pe restrict kar dia gaya,but ye 160kmph pe bhi chal sakta hai.

Aerodynamics is not an issue with these locomotives. :)
For a locomotive hauling a train, the wind hitting from front is not a problem until the train attains high speeds (almost 180-200 kmph). And for most of the time, the cross winds (wind blowing under the coach, between the coaches) and the track resistance are more critical for speed. Even EMD's do not have a special aero-dynamic design. while the short hood side in EMDs has a cab with rounded features, it only offers a very small reduction in the air resistance, and on the whole, does not make any difference to the acceleration or top speed of the loco. When running in LHF mode, even EMD's do not have...
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an 'aerodynamic' front, but they are equally capable of 160 kmph in both LHF and SHF. The restriction to 110-120 in LHF mode is only because of visibility problems for the LPs in LHF mode.
Air resistance plays a very negligible part in the acceleration of IR's locomotives. While the WAP1/4/5/7 have a 'V' shaped nose, for the speeds at which these run, this really does not affect the resistance much. And the air resistance at the front is not strong enough to reduce the acceleration of the locomotive.
11121 ( ED EDM3D) had an EMD style cab as IR was trying to mimic the EMD style control stand and electronics in the WDM3D. This was an experiment at improving the visibility for LPs in the short hood and provide a modernized control stand. This had nothing to do with aerodynamics or MPS increase.

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.

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'...
more...

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.