Originally Posted by
stimpy
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.
I know this is common practice and risk of getting a fine is minimal, but it is formally not allowed to get off the train and continue on a later train. If your ticket has been checked on the first part of your trip it will be marked and you can get caught. The proper way is getting two separate tickets or a "Via" ticket (Brussels - Amsterdam Via Antwerp).
Also: if merely passing by Brussels Airport (not getting off) you will not need to pay the supplement so don't let that limit your choice of trains.