I agree with the previous reply. It's always better to leave half an hour for imm.check and for us the thing was they only let you board the train nearer the time it leaves, so be aware that the train is long and you might have to walk about 5-10 min to your carriage (as the previous poster said). we got into trouble as we got panicked and got into wrong one and got down again etc...
This is far better than the airport checks and even the journey is so pleasant, the food is a bit pricy so if you are really skint, a tea flask and sandwiches is a must to take in. The midi station in Brussels is not the best compared to UK.