You could go by TGV from Milano or Torino to Paris
and catch then a Thalys to Brussels (both trains need extra reservations ontop of the Railpass and these are quite expensive)

To save money on reservations you may go via south france. and catch from Nice to Paris a domestic TGV 10€
and from Paris to Brussels you may go via Lille and connect there further to Brussels as it´s cheaper as the direct Thalys.