If you are looking to travel by Thalys to Brussels or Amsterdam, I suggest the Netherlands site. https://www.nsinternational.com/en/traintickets-v3/#/preferences/FRPNO/NLASC?utm_medium=ns.nl&utm_source=planner
Why?
1- you can sign up for email notices when your preferred travel date is available for booking. The sooner the cheaper!
2- NS only requires inputting traveler's names vs. name, email and phone # on Thalys, which means a tedious drop-down to get to the US +1.
Don't forget the sooner you book the cheaper the fare.
Thanks to an email I received this morning, we are good to go for late August. Based on research I was hoping that bookings would be open later this month or even later. It was a surprise that our date was now available, but had I not signed up thru the NSI site, we could have been spending more for our fare by booking later.