I just love long rail trips, often do Switzerland-Tunis via Palermo to see my in law family. Best trip ever was Tbilisi-Zurich via Batumi, ferry to Odessa Kyiv and Vienna.
I wish to see high speed maglev transportation to supplant air travel in my lifetime.
Other countries I'm familiar with (Spain, France, Germany, UK, Japan) certainly do let you turn up, buy a ticket, and get on. It will cost more than buying earlier.
In China, ID and ticket are the same thing. You know that already.
Europe and Japan (what I meant by "rail heritage") are indeed more relaxed. But even in Europe the newest high-speed network, Spain's, follows the airport template, with security check and departure lounge. You have to turn up 25 minutes ahead of departure. It's such a shame and so self-defeating.