> The presence of fast trains between two cities, on the scale of <600km, is transformative, as it becomes more convenient to take a train than to fly

I haven't found this to be true for London to Brussels. Eurostar terminals seem to operate like airports and require check-in two hours in advance, with airport style security. So I don't save time, and for me, my regional airport is much easier to reach than the Eurostar terminal.

I can believe it works better for "normal" train travel within the Schengen area, but I wonder: what are the security requirements then, if any?