Originally Posted by
KQ321
If Brussels-Amsterdam is on the InterCity (not Thalys) then the ticket should be valid on any InterCity train on that route for that day (ie: it's not tied to a specific train). However, I am not sure if this allows break-of-journey en route.
What the OP wants to do is just fine and he won't have a problem. With normal trains you can hop off mid-way and hop back on again later that day without a problem. If you buy a ticket from Brussels to Amsterdam for a particular day, you can take any normal non-supplement, non-Thalys train route you wish to.