I found recently through TSE the existence of the trampoline ticket. A ticket that allows you to get to Lille, from some selected stations in Belgium, with unlimited travel.
I only found information about it in the Belgium rail website (http://www.belgianrail.be/fr/titres-de-transport/transfrontalier/lille/trampoline-1-jour.aspx). Is this a train only ticket? is it a regional ticket? What does unlimited travel mean? Does it mean only on train or does it include other transportations?