When it stands that you need a reservation, than you’re NOT allowed to hop on a train without it. 

Which train do you want to take?

Thalys, eurostar and SNCF

Thalys and Eurostar always need a reservation you won´t be able to enter without or have to pay high penalties :/

SNCF is just the national rail carrier we need to know which Train category (TGV,IC,TER) or even better the Routes you are asking for For example Amsterdam - Brussels you can avoid thalys by reservation free Intercity or Brussels - Cologne you can avoid Thalys by reservation optional ICE :)