Upvote:2
Typically, if you don't board the initial flight (or rail trip in this case), an airline ticket's reservation for all subsequent travel is cancelled.
If the train is an early-morning train from Brussels, your best bet is probably to take a train to Brussels the night before, and to wake up in Brussels. Either that, or call your airline to change the booking (or to do so online) and see if the cost to amend the ticket is reasonable.