When I was in Central Europe and wanted to go around to other countries, I panicked about the train tickets and wanted to buy it online and in advance... but my course advisor (I was there on exchange) said not to bother, just go to the station 30min in advance and buy it. He turned out to be quite right. He also said there was no need to buy the seat reservations, and we didn't at first, but after the first ride, we immediately bought reservations for the return journey.
What happens as far as I can tell is that they just keep selling tickets. The train gradually gets more packed as it goes along, and if you get on at one of the later stops and don't have a seat reservation (which is purchased separate from the ticket), you're just going to have to sit on one of the fold-down seats along the hallway, or on the floor, or inbetween carriages, or stand. With your luggage.
My friends and I were lucky - on our first ride, we got on at the very first stop. So we didn't have any problems finding seats that hadn't been reserved. But we saw what happened afterwards, and since on the return journey we would be getting on halfway and it was a long overnight ride, we bought the reservations.
What happens as far as I can tell is that they just keep selling tickets. The train gradually gets more packed as it goes along, and if you get on at one of the later stops and don't have a seat reservation (which is purchased separate from the ticket), you're just going to have to sit on one of the fold-down seats along the hallway, or on the floor, or inbetween carriages, or stand. With your luggage.
My friends and I were lucky - on our first ride, we got on at the very first stop. So we didn't have any problems finding seats that hadn't been reserved. But we saw what happened afterwards, and since on the return journey we would be getting on halfway and it was a long overnight ride, we bought the reservations.