>What I was asking was when is the train used. It appears to go to/from the airport and to different cities.
Metro, tram and bus. These appear to use the same ticket system. I'm still a bit hazy on what to use here.

Yes, you are right about the train - there are comparatively few train stations around Brussels and obviously you can travel between those stations inside Brussels if it is convenient to you but mostly it won't be (airport train, train to Ghent, Antwerp are exceptions) and the metro (densely spaced stations, many, many of them in Brussels and up to its outskirts) will be far more convenient. You cannot use train tickets on the metro or vice versa. The train is mostly to get you to destinations (cities, towns) outside Brussels. You cannot reserve seats and a ticket is good for any train on the same day.

Yes, metro train and bus use the MOBIB card tickets.

Lavandula