I live in Switzerland where people are so comfortable taking the train they treat it like an extension of their living room.
Only in rare cases do I even book tickets in advance, like when going to Milano… otherwise I just use the Fairtiq app, which is a nation wide system for paying for tickets, including busses and trams…
You swipe right before you step on, swipe left when you step off and the system automatically calculates the best ticket for you.
There isn’t a “fully booked”.