Free train travel in Belgium was once a nice perk of taking the Eurostar to Brussels.
If you took the Eurostar before 2011, you could travel for free to any Belgian train station on the day of your arrival. The same was true for the day of your departure if you left Belgium via Eurostar.
Unfortunately, the free train travel scheme was scrapped in late 2010 and replaced with a supplement that covers onward travel to any station in Belgium.
Thus while travel is no longer free, it is still less expensive than purchasing a second ticket on most routes.