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.