Which route?
In theory, you should always take the shortest distance between departure and arrival stations.
But you can take a different route if it'll get you to your destination faster. However, you can never start further back than your starting point or go past your destination point and back unless you have paid for these journeys.
If you'd like to extend your journey beyond the station shown on your ticket, buy a top-up before leaving the station (you can also buy cards for 10 top-ups).
More information
There will be an additional administrative fee of 3,00 EUR* for a top-up bought on the train if tickets were on sale at the station where you originally boarded.
Remember ...
If you haven't got a valid ticket, always tell the train staff before boarding or immediately after setting off. Otherwise, you will be treated as an unauthorized passenger.
Domestic tickets are only valid on the Belgian railway network.
* Prices valid from 1st February 2012