Advice for Traveling from FCO to Montalcino.

You would take the train to Buonconvento and then get the bus https://www.frommers.com/destinations/montalcino/planning-a-trip/getting-there

Or just rent a car.

The best way to get from FCO to Montalcino is by rental car. It's a place that's really off the beaten path.

When we travel to such places, the only way to see the countryside is by rental car anyway.

Yes, rent a car, easiest way to get to wherever you want to go.

We rarely rent cars but for rural Tuscany we make an exception. Otherwise you are subject to infrequent busses, taking tours, or hiring taxis.

hey hey mike
i agree with others to get a rental car to get around tuscany towns/villages. some buses and trains don't go near the places.
if you do rent a car make sure you get an IDP (license permit) at AAA.com
read up on it/
aloha

Note that the roads in Tuscany are well paved and well marked. They're just a little hilly and crooked. Not at all a hard place to drive in.