the fastest diesel locomotives usually have a speed around the low 200kph range, while the fastest electric locomotive can go over 300kph, and up to 574 kph. what is the difference? what makes it not currently possible to design a diesel locomotive that can go over 300kph?
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It’s likely the “tyranny of the rocket” problem.
Carrying fuel is not free. Diesel has mass and a diesel locomotive must expend fuel in order to accelerate that fuel. The more fuel you carry the more fuel you must carry to move that fuel.
Moving faster costs more fuel for the same mass, which means you must carry more fuel to move that extra fuel, which requires more fuel to move.
Eventually you reach the point where you have to carry more mass in fuel+vehicle than the total mass you can move with that fuel. I’m sure trains, even going 574 kph, aren’t nearly to that point, but they’ll still reach the point where the train is more fuel is a high enough percent of the total payload that it’s simply not economical to do so.
Electric trains, on the other hand, don’t carry their own fuel and so aren’t limited in that way. It still costs more energy to move fast than to move slow but since they aren’t carrying their own fuel the energy costs don’t ramp up nearly as fast, allowing for higher speeds to still be economical.
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