How long (hours) will it take by train from Brussels to Lake Como & how many times will we need to change trains? Can we sleep overnight on the train if it is a very long journey? What is the best & fastest route?

you can check train schedules at www.bahn.de. click on "international guests" on the top to get an english page.

The easiest connection according to www.bahn.de seems to be this one:

Brussels 17:28 - Koln 19:45 by Thalys
Koln 20:18 - Como 07:01 by nighttrain

The total journey time is 13:33 which is long!

Why don't you take a cheap flight to Milan and a train to Como from there? It's probably going to be a lot cheaper than the train as well. Or do you really want to go by train?

For about 40 Euro you can fly from Brussels International airport to one of the Milan airports via virgin express airways (http://www.virgin-express.com/skylig...mp;page=search).
They have an early morning (7:15) and a 6pm flight schedule.

The night train from Köln with sleepers and couchettes looks to be the quickest to me - any daytime flight will actually take more time. I'd suggest travelling earlier from Brussels to have dinner in Köln before boarding the train, or spending some time in the Rhine valley before boarding the night train to Como at Koblenz or Mainz.