AFAIK, all high speed trains (200+ kmh) run on electric track.

Why is this? Is there something about a diesel-electric engine that makes high-speed trains more difficult?

Here's some other data, and correct me if I'm wrong:

Electric track is high-maintenance. Using a pantograph to pick up power from overhead wires is also non-trivial at speeds of 200 or 300 kmh. The electric system is much more prone to failure in bad weather (storms, snow/ice, etc.).

I've often heard the claim that electrified rail is more energy-efficient than a diesel-electric. However, I've never seen numbers showing exactly how much more efficient it is compared to diesel-electric. Maybe someone knows a source on that.

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    $\begingroup$ Not quite true : en.wikipedia.org/wiki/British_Rail_Class_43_%28HST%29 which does explain some of teh constraints. Low axle weight is probably easier in a locomotive with the prime mover somewhere else. $\endgroup$ – Brian Drummond Oct 22 '16 at 10:20
  • $\begingroup$ @BrianDrummond Thanks I will read about that engine. Low axle weight is probably easier in a locomotive with the prime mover somewhere else. Yes, but only for the engine car(s). In modern diesel-electrics, the electric power is sent to the wheels of many or all of the car bogeys. In other words, each car has its own electric motors driving its own wheels. This is true in a modern diesel-electric and a fully electric system. So the cargo cars have the same weight. $\endgroup$ – DrZ214 Oct 22 '16 at 23:26

Diesel locomotives can't deliver enough power. The power rating of the diesel generator on most locomotives ranges from 2 to 4 megawatts. The combined power rating of motors on high-speed trainsets range from 5 to 20 megawatts, depending on size and design speed.

Diesel engines are rated for maximum power, while motors are rated for continuous power. Motors can deliver much more than their continuous-rated power for a few seconds. This means much greater acceleration, and much greater power requirements, than any engine could provide.

Note that aerodynamic drag, which is the dominant resistance on all but the slowest trains, is proportional to the square of speed.

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    $\begingroup$ Thanks. Can you post a source for the 5 to 20 MW for high speed trains? I would have thought a diesel pulling a long freight train could pull maybe 10 passenger cars, which are much more light, at 2x or 3x the speed. $\endgroup$ – DrZ214 Apr 2 '17 at 7:56
  • $\begingroup$ @DrZ214 the mass (or weight) of the train has no effect on the maximum speed, though it does affect the rate of acceleration up to the max speed. Much of the aerodynamic drag is caused by the frontal area of the train (i.e. by the size of the locomotive), not by the length of the train $\endgroup$ – alephzero Apr 10 '17 at 12:34

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