What is the best way to go from Brussels to Trier for an overnight trip?

You can do it in about 4 hours by train. Go to bahn.de to find a schedule.

Brussels direct train to Luxembourg City then change to hourly or so trains to Trier. Should take about 4 hours but check nytraveler's bahn.de site for schedules.

If possible take a quick look at Luxembourg City enroute, one of Europe's most physically gorgeous cities IMO as a deep gorge bisects the city.

Don't take the same route coming and going, make the most of it.

Go via Luxembourg (and, indeed, take a few hours to break the journey there), and go back via Koblenz (some Trier-Koblenz trains are fast, like the 08:13 that gets to Koblenz by 09:38).

Trier-Koblenz has very nice scenery.

Change trains in Koblenz for nearby Cologne and break the journey. The Cologne cathedral is across the square from the station, and the chocolate museum is on a half-island in the river - www.schokoladenmuseum.de - yummy!

On your way back to the trains, drink the famous "Kölsche" - the blond beer so beloved by people in Cologne.

Then hop on the fast train to Bruxelles.

Make your reservations early so you'll have seats on trains spaced far enough apart to allow these gallivants - it will be a much more memorable trip than just going to Trier.

Then hop on the fast train to Bruxelles>

there are German ICE trains and Thalys trains serving this route - ICE trains you can just hop on with a ticket (or railpass) but Thalys demands reservations before boarding and it also have a Byzantine fare structure - just show up and you may pay a whole lot more than booking in advance at www.thalys.com

Try to rendez-vous with the ICE trains IMO as they simplify everything so much

But i do like DalaitLama's thinking there - just seeing the Cologne Cathedral up close is worth the trek.

PalenQ is right, that's why I typed in my previous post "Make your reservations early ..."