The distance between
Brussels and Amsterdam is around 200 km. It will take just over 2 hours
if you drive by car.
By high speed train both city centers are perfecly connected several times a day in 2009.
So why fly? Well, Amsterdam Schiphol airport
(AMS) has much more
intercontinental flights than Brussels
KLM has many daily flights from Brussels to Amsterdam often with their Fokkers in 2009.