What is your travel date ?

The Thalys Paris - Brussels has an expensive seat reservation of 27€ that can sell out weeks in advance so it's better to avoid it.

The direct TGV Montpellier - Brussels (20€) has a quota and seats sell out weeks in advance. You could take the same TGV to Lille (10-20€) though and then take a reservation-free regional train across the border

I want to go on the 5th of May and want to come back on the 7th of may

Oh i didnt know that… If i cant book a seat i cant go back home, can i get my money back for the pass? 

There are seats to and from Lille (20€) so it's still possible to travel, don't worry. Unfortunately at the moment TGV to Belgium can only be booked by calling SNCF or visiting a SNCF train station.

You could get a 85% refund.

It's possible to book here for TGVs within France : https://travel.b-europe.com/Eurail-GE/en/booking-tgv#TravelWish (4€ booking fee per order)

Btw an Interrail pass can only be used on 2 days in your country of residence.

On 5th May there are no 2nd class seats left on the afternoon TGV to Lille (even regular tickets). Here are possible options :

- TGV Montpellier-Sud de France - Lille-Europe 09:12 - 14:24 20€ (16:49 in Bruxelles-Midi via Tournai)

- TGV Montpellier-St-Roch - Lille-Europe 14:51 - 19:58 40€ 1st class (seat reservation + surcharge for a 2nd class pass) (23:14 in Bruxelles-Midi via Mouscron)

- TGV Montpellier-St-Roch - Lille-Flandres 05:28 - 10:42 10€ (13:14 in Bruxelles-Midi via Mouscron)

Fridays and Sundays are two busy travel days so yes seats can sell out…

(Thalys Paris - Brussels is alread sold out)

Edit : si tu es à Montpellier, tu peux aller demander demain matin au guichet à la gare St-Roch. Ils pourront te dire s'il reste des places pour Bruxelles (ce qui m'étonnerait étant donné qu'il n'y a presque plus de sièges disponibles le 5 mai) et te vendre les réservations pour Lille (ou éventuellement Bruxelles).

Il y aura moins de problèmes pour le retour

You can also book all these trains at an SNCF or NMBS/SNCB international ticket office, without any booking fees.