I will be traveling to Europe....do I need to buy rail passes now, or can I buy them at the train station?

I will be visiting Barcelona, then Paris, then Amsterdam, then Prague, and then Florence. From Barcelona to Paris I plan on flying, but to get to the other destinations I plan on taking the train. Do I need to buy a train ticket in advance, or I can just go to the train station and buy them when I am leaving?I can't find ticket prices, can someone find a link for me please??