Do I need to buy a Thalys train ticket in advance?
You can buy tickets at the station. HOWEVER – It is generally adviseable to get them in advance. Not only is your ticket your seat reservation, it is usually cheaper, as Thalys uses market prices – meaning that a ticket can be changing prices daily. As ticket = seat reservation, you can`t get open date tickets. I would suggest that, as soon as you know WHEN, you make the reservations online.