有効です

下記の通り

www.b-europe.com/Travel/Practical/Any%20Belgian%20Station


THALYS TICKET WITHIN THE BRUSSELS, ANTWERP AND LIÈGE ZONES

Thalys tickets from or to Brussels-Zuid/Bruxelles-Midi (Brussels) are valid within the entire Brussels zone. There is no need to buy another ticket if you are connecting with your Thalys service, as your Thalys ticket covers train travel within the Brussels urban zone.