Yes it is valid but you need to pay a 32€ seat reservation which can sell out weeks in advance.

There are usually more seats to Brussels + reservation-free hourly IC train.

Or another alternative, it takes longer but there are hourly trains :

- TGV to Lille 10-20€

- IC Lille - Kortrijk

- IC Kortrijk - Antwerp

- IC Antwerp - Amsterdam

Check Thalys (to Brussels and Amsterdam) availability here : https://www.b-europe.com/EN/Booking/Pass#TravelWish

Let us know your travel date and we can help you