Another dimension to this is our demand for speed. Why is it so important to go so fast? A conventional train that terminates in the city center eliminates the commute to the airport, and weather-related delays are fewer with trains, so the average total trip time should be about the same for many routes even if the top speed isn't so high.

I was reminded recently that during the Second World War, the national speed limit in the U.S. was 35 MPH. You can go a long way in a day at 35 MPH...