@JubilantJaguar @balsoft yeah, hydrogen planes might be needed. But I think there is a good chance that we just aren't going to have a wealth of choices for Australia to america, etc.The economic option will be more like a 2~3 day journey that only flow where it must and goes most of the way via train. Australia to the US might be via Alaska for most budgets.

We would have to make it sustainable eventually, since it's the only practical way for passengers to travel between americas/australia/afroeurasia. I guess something hydrogen-based is the most likely candidate for reducing the carbon impact.

Hoovering and hydrofoils have been under-explored, but yes, speed is necessary for long-distance travel.

all high-speed trains are running on 100% renewable

Irrelevant to what I said: discounting energy source.

Trains obviously have much less rolling resistance, as you say (an advantage partly offset by their added weight). But wind resistance is the bigger factor, and trains are usually just faster.

A train travelling at 350kmh uses 40% more energy than one going at 300kmh. This is why the service speeds usually top out at 300 everywhere, Europe, japan and China.

Speed really is the decisive factor. The pyramid hints quite well at this.