Rome to Sienna

One of the wonderful things about Italy is the quality of public transportation. Yes, you can get to Siena from Rome by train or by bus. Check both schedules (more for length of journey than anything else). How tired are you going to be after your flight? The direct train (at least one, maybe more, changes) and about 3 to 4 hours of travel doesn't sound so direct to me. This is going to sound longer but it really isn't: take the Freccia from Roma Termini to Firenze S.M. Novella. Walk across the street and catch the bus to Siena. It's a lovely drive through the countryside and you can see several hill towns. Travel time is going to be about the same! The train part is 90 minutes on a very comfortable train. The bus will drop you off right next to the historical, medieval city you want to be in. I took this route a few years ago and found that the comfortable seats on the Freccia were very welcome!

If the times work out for you, take the Cortal bus to Rome Tiburtina station. If this doesn't work, take the train from FCO to Tiburtina. Then the Balto bus to Siena. This bus runs hourly.

Edit. Baltour bus.

Since you are passing by you might also consider taking the train from Rome to Orvieto and consider staying in that area first. If you have a car (or you can take the bus from the train station) you might consider Bagnoregio, Lubriano, or one of the other nearby towns..sounds like you may already have a plan? Sienna is not far by car or bus....or upon leaving Orvieto take the train to Firenza and the bus from there. The bus station is just outside of the train station...

My plans have been made & is as follows: arrive in Rome, get to the main train station & take a train to Orvieto & stay a while.

I also plan to spend some time in Perugia & am looking into renting a car in Orvieto. I still have to call Hertz direct for that (Autoeurope & Kemwell were not able to find cars in Orvieto in their system), backup plan is public transportation to Perugia.

I will be returning back to Rome later in the month from Sienna & it looks like that is possible by train with one connection. Not sure yet if this connection should be in Poggibonsi or if there will be more schedule options someplace else.

And I have already gotten my international drivers license from AAA.

I would have also suggested going to Orvieto first, so it's great to see that you've decided to do that. You may not need a car for the first few days, depending on where you plan on touring in that area. Spend a day or two exploring Orvieto while you get over jet lag.

In addition to the IDP, be sure to do some research on ZTL (limited traffic) areas, as those exist in many Italian towns. Hefty fines for each pass through one of the restricted areas! Also, parking in Siena can be difficult at times.

The rail station in Siena is at the bottom of the hill, so you'll need to take a Taxi from the town to connect with the train. If you wanted to take a Bus back to Rome, the Bus station is in Piazza Gramsci in the centre of Siena, which is a bit easier. Trains on that route often seem to connect to a Freccia in Florence.

You may already be aware of this, but when you first arrive and take the train into Rome, be sure to validate your ticket prior to boarding the train or you'll risk hefty fines which will be collected on the spot! Also note there are two trains serving the airport, the Leonardo Express and the FL-1. Be sure to board the correct train! Validating locally purchased tickets applies to Metro, Bus and other forms of transit as well.