Montecatini Terme is on the train line that connects Lucca to Florence which are then connected to the rest of the rail network in Tuscany and Italy - you can basically get everywhere you want to go to in Tuscany from Montecatini with changes in trains in Florence.
You can see a map of the main train lines in Tuscany on this page: http://www.discovertuscany.com/moving-around-tuscany/by-train.html
Depending on where you want to go, you might have to change trains but you can still get there if along on the train lines. Some great day trips:
Lucca, Pisa, Florence, Arezzo
Arriving in Florence, to reach Siena the best thing would be to continue on SITA bus since it is more direct.
To see timetables, go to www.trenitalia.com