And how do we cross the oceans?

Let’s see even a high speed train cross the US, from LA to NY. Every state will want it to stop in at least one major city, or they simply won’t vote to subsidize it.

Not to mention the land required - find a way to acquire the land between LA and NY to support it. Same problem as above.

All this without even approaching the flexibility issues, and many more.

While planes can go wherever, so long as the airport supports that class of plane (length/weight).

Trains have their place, in well-established routes that don’t change much. They can tolerate increased use to some degree, but as soon as you lose ridership, costs per traveler balloon.

And they still require many tons of concrete, as the bed for high speed has to be even more stable than for an interstate.

And, you’re still accelerating (start/stop) many tons, regardless of how many passengers are on board.

Trains aren’t a panacea, not a free lunch.