I'll be arriving at AMS and taking a train straight to Brussels. The NSInternational site seems to sell tickets for specific trains (NS trains, not the Thalys, which are also listed), but elsewhere there's language that a ticket is good for any train on that date.
Can anyone confirm that I'm OK to pre-purchase the fare without knowing the exact departure I'll be able to get on? (As that will depend on how quickly Schiphol spits me out.)