> Wouldn't the hassle in coordinating things across multiple countries be much higher?
Not much more than across US states. https://en.wikipedia.org/wiki/Schengen_Agreement
(although as someone else commented, it's not easy to get trains going through more than one border).
> Or ignore it and wait for automated bus services.
Buses won't reach 330 km/h (210 mph, speed of trains made in my country, Spain).