However, If the Intercity Direct supplement for the stretch between Rotterdam and Schiphol is not included and if you want to avoid the 10 euro fine if you have to buy it on the train , you'll have to get off to buy the supplement. I.e. go down to the station hall, buy the ticket at the ticket window, and you'll have to take the next train. Hence my comment on making sure the supplement is included in the price of the ticket you buy in Brussels.