I have traveled by train in Europe before, but mainly in the Czech Republic, Germany and Italy. Next month I'm in Greece and I have plane tickets to Athens and from Thessaloniki. In each city, accommodation is also booked for several days. This means that I plan to get from one place to another on a certain day, but I have no particular time restrictions.
Should I book a ticket in advance or is it okay to wait until the day and buy at the train station? If I book in advance, do I commit to a specific time now?