Train to Madrid

Trains from Brussels Airport train station to Madrid take 13.5 hours with 3 connections. You must leave by 10:30 a.m. to make it in one day and trains are expensive, unless reserved one to three moths in advance. It would be faster and cheaper to fly; see www.skyscanner.com.

How to Look Up Train Schedules and Routes Online gives you the DB train schedule link and tips for using it. This will also show connections from Brussels Airport train station to any destination.

Let me get this right, you are landing at Brussels at 08:30 and immediately taking a train to Madrid!
That will take over 13 hours and involve 4 trains. There is a station at Brussels airport (as there is at most airports).

Bruxelles-Nat.-Aéroport depart 09:57, train IC 2831, arrive Bruxelles-Midi 10:17
Bruxelles-Midi depart 10:29, train TGV 9826, arrive Lyon Part Dieu 14:00
Lyon Part Dieu depart 14:30, train TGV 9743, arrive Barcelona Sants 19:28
Barcelona Sants depart 20:00, train AVE 3202, arrive Madrid-Puerta de Atocha 23:10

But why? Why don't you just fly direct to Madrid, or take a cheap flight from Brussels to Madrid, that would be cheaper and quicker.

Thanks Chris and Laura,
Actually we like riding the rails as we have done it many times before. But I appreciate all the info.