Hello! Im thinking to side trip to Pisa during my 3 night stay in Rome ... can anyone advise on the how to do's for this thought? What train can I get and where to buy tickets???

Also I would be having a 3 day train pass for Eurostar -- is this train pass good as a ticket? Can I get this ticketed at Rome Termini where I will be boarding for Mestre?? Shall I do this in advance online or can I do it at the time of my actual travel at the Termini???

Appreciate your inputs please =)