I've traveled by rail in Europe before, but mostly in the Czech Republic, Germany, and Italy. Next month I'll be in Greece, and I have air tickets into Athens and out of Thessaloniki, with accommodation also booked for several days in each city. That means I'm planning on getting from one place to another on a certain day, but I don't have specific time constraints.
Should I book a ticket in advance, or is it okay to wait until day-of and buy at the station? If I book in advance, am I committing now to a specific time?