The best part of interailling for me was turning up to a major train station, looking at the giant departure board, choosing an enigmatic destination and just getting on the train. No booking or reservations or even planning at any point.
Is this not possible any more?
Yes. The app makes it really easy to do that. In our last trip we did it a few times.
But, as you get older, there's a certain joy in making plans in advance.
I used to do this but with Flixbuses and Ryanair. Back then one could fly from Frankfurt am Main to Milan for 15 euros.
For faster/intercity trains, it is still like that in Switzerland, Austria, Germany, The Netherlands, UK, Ireland, Denmark, Norway and most of central and eastern Europe [0]
France, Italy, Spain, Portugal & Sweden require seat reservations, as do most international services
Making reservations varies from easy to a complete pain
Quite a few countries (France, Spain, Italy for starters) require seat reservations in advance for any long-distance/high-speed trains now. And worse, they have a quota for passes, so even if the train has space, you might not be able to reserve a seat and therefore get on the train. A real shame as it makes it far less flexible than before.