Tuscany towns by train? Or do we need a car?

You can get to Lucca and Siena by train. But you'll see more of Tuscany and the hilltowns with a rental car. I just saw that you posted on the IDP post as well and that you are thinking of renting for a day or two. That will be more expensive per day than if you are able to rent for a few days more.

But splurge for the rental car if only for two days and you will have a wonderful time in Tuscany.

From Florence, you have great train service to Siena and Lucca. Once in Siena, there is good bus service to Montepulciano, San Gimignano, Volterra. For really good movement around Tuscany (i.e. small out of the way areas, wine tasting, etc.), a rental car is a very good choice.

I don't know how many days you are talking about, but in the past I have run into the problem that the rental company has a minimum number of days that they rent for. If you rent for fewer days than the minimum, you are still charged for the number of days they consider a minimum rental. Unfortunately, I can't remember what the minimum was. And it may vary by rental company.

That being said, I think you will not regret it if you get a car. The Chianti region is beautiful and you will probably want to drive to Greve or Radda, or do a circuit through there. I think it is the DK guidebook that has a nice circular route. Also, the drive from Montalcino, through Pienza and to Montepulciano is one of the most beautiful drives I think I have ever taken. If you make that drive you will see and probably immediately recognize two of the most photographed scenes in Tuscany. A small chapel with a round window over the door and cypresses on either side and a clump of cypresses that I think must be in every picture book or calendar about Tuscany.

I'm going on a trip there in early May, and though I would love to rent a car & spend the entire time in the Tuscan hillside, there's just not enough time for my trip. I am planning a trip to Siena, and if you opt out of renting a car, I've heard that taking a bus to Siena is much better than taking the train. The bus will drop you off in the town center, rather than the train, which drops you off down a rather large hill from the town center. I have a fairly recent thread started called something like "Day Trip from Florence: Rome or Siena," which has some good information on buses & trains to Siena.

I've been studying the Siena bus schedule and it's nearly impossible to make a day trip to Montepulciano or Pienza if you want to stay more than a couple of hours. The last bus out from Pienza leaves in mid afternoon--- the first bus in arrives about noontime.

The Lucca and Siena aren't villages but major cities. As such they can be accessed by bus or train pretty easily. But as others have mentioned, Tuscany is one of the regions in Italy where a rental car would give you much better access to the backcountry and the truly unique villages. And it's not that difficult to drive in this region.