purchase a 6 day rail pass or point to point tickets Traveling to Rome-Venice and returning to Rome Rome-Florence
Florence-Sienna
Point to point.
Linda, Although I haven't done a detailed cost comparison, generally speaking in Italy P-P tickets are the best choice. On the main routes (Rome-Venice, Venice-Florence, etc.) you'll probably be using EuroStar Italia trains so reservations will be compulsory. With a Railpass, you'd need to pay the reservation fees separately. Happy travels!