Reservations are a must especially toward end of August when everybody starts coming home.

Also be prewarned. Your train may stop in the middle of nowhere and just sit there for hours. Why? Strike and related work stoppage. I don't knopw if it still happens but it cured me of ever wanting to ride the rails in my country. I don't know if there is way of finding out if work action is scheduled but it would be worth checking in to especially if you have flights to catch.