The Italian railway system, run by Trenitalia, is connected with the rest of the European rail system so it is easy to reach Tuscany from locations across Europe and Italy.
Overnight hauls give you the possibility to book a couchette (cuccetta) on most international trains. It is always advisable, and sometimes required, to book seats on international trains to and from Italy.
The main route that passes through Tuscany is through Florence, on the Bologna-Rome line via Arezzo. Another line from Rome hugs the Tyrrhenian sea coast and passes through Grosseto, Livorno, Pisa, Viareggio and on along the coast on its way to La Spezia and Genoa (in Liguria).
Florence's main train station is Santa Maria Novella (SMN). Direct international trains arrive here from Paris, Calais (during the summer), Brussels, Amsterdam, Basel, Munich and other European cities. Fast Eurocity connections arrive from Rome, Milan, Bologna, Pisa, Verona, Venice, Genoa and Turin.
You can check timetables on Trenitalia. It is not worth buying Eurail or InterRail passes if you will only be traveling in Italy, as train fares are reasonably cheap. You can buy rail tickets from most travel agents or directly at train stations.