You basicallly have two(2) options for train travel between PARIS and MILAN. The first are the several (7 trains--3-direct, 4-one train change) high-speed, TGV train connections during the day departing from PARIS--Gare de Lyon to MILAN--Centrale. The average train travel time for direct (no train chages en-route) connections is approx. 7 hours. You other choice is a slower, direct (no train changes en-route) night/sleeper train departing PARIS--Gare de Bercy at 22:20 (10:20 PM) and arriving at MILAN--Centrale at 08:59 AM the following morning--approx. 10:40 hrs overnight, travel time. For details on schedules, fares, discounted fare options, advance ticket purchase (usually involves a substantive cost saving the farther in advance you book), check the SNCF (French Nationl Railways) web site at:
www.voyages-sncf.com/dynamic/_SvHomePage