Tuscany without a car

Yes, probably. Siena is a hub of bus lines and therefore a practical homebase for the traveler not renting a car. It is also fairly centrally located in the "Tuscany region", depending on how you define that region. Trains link some of the towns; but hill towns -- being on hills -- don't quite fit the railroad plan. Train stations are likely to be in the valley a couple of miles from the town center, usually connected by local bus. A problem with buses is Sundays and holidays, even from large cities like Siena, bus schedules are sparse on those days and ticket offices are often closed. Plan ahead and buy your tickets in advance if you're going to travel on Sundays or holidays. To really explore the Tuscany countryside with flexibility and at your own pace you would need a car; but then the driver does not get to see the scenery because he or she is keeping eyes glued on the road (Italian driving is another subject, well discussed on the HelpLine).

Kent, we almost learned that the hard way in Siena. Our departure day was a Sunday and thank heavens we could speak enough Italian with the owner of the B&B where we stayed so that she could order us a taxi to get us to the train station in time for our train. As you point out, the train station in Siena is nowhere near the historic center. I've noticed the same is true of Assisi where we'll be going in May.

We stayed in Siena proper the two nights we were there and didn't do any daytrips because we were enjoying Siena so much.

Prior to Siena we had stayed at an agriturismo outside Arezzo (not far from Cortona). It was the only section of our month long trip where we were glad we had a car. It allowed us to go quickly to Arezzo, Cortona, Sansepolcro and Anghiari. With a car you get flexibility but the challenge of driving on Italian roads with Italian signs and Italian drivers. Buses take much longer, but give you the opportunity to interact with locals and to enjoy the views as you go (which in Tuscany can be breathtaking).

Dianna, Siena is ONE of the best locations, but as previously mentioned the railway station is "down the hill" so not as convenient for day trips. This requires a Taxi or Bus.

One thing you might consider is private tour companies operating in Siena, as these will provide transportation to local sights. As I recall, there are several mentioned in Rick's Guidebook.

Having a car is not necessarily the best idea. When I was in Siena last fall, I spoke with one couple that spent two hours trying to find a parking spot. Their experience might be the exception rather than the rule, however I find it much easier and less complicated just using public transit.

Which city will you be in visiting prior to Siena? If traveling from Florence I found that it's best to use Coach to Siena, as these arrive in the centre of town.

If there are any unusual and "out of the way" places you want to visit, you could always rent a car for the day. Some rail passes provide a "Rail & Drive" option.