Another useful tip for people traveling domestically by train in Belgium might, depending on how many legs you actually travel and how many people you are, be to consider buying a RailPass, which gives you 10 person-journeys at a very affordable fixed price of 77 EUR in 2nd and 118 EUR in 1st class, and may be a bit cheaper than buying individual tickets, even if you don't use all trips.
Once you buy the pass, you'll receive a ticket with 10 blank lines. You simply fill in one line per person and trip, the entire thing is valid for all connections within Belgium except for ICE and THA rides.
More information is available here:
http://www.belgianrail.be/en/travel-...rail-pass.aspx