I am thinking that after 2 days in Rome we will go to Florence and make that the base for 2 days. THEN we can spend the next day and night visiting San Gimiagno and staying in Siena. My question is - can we take the Eurostar from Siena to Naples (to connect to Positano). Another possibility is to rent a car from Florence to drive to Rome with stops in SG and Siena but my husband said the rent in Florence/drop off Rome will be an added expense and it might be better to bus to the towns and train from Siena if possible. I hope this is clear! (as mud, right?!)

Start over and give the basics and perhaps we can help.

Thanks, Bob.
1. Is there a Eurostar train from Siena to Naples?
2. Are we better off renting a car at the end of our stay in Florence to drive back to Rome via San G and Siena for a day and night * OR * taking the bus to Rome from Florence with stops in these towns for a day and night and THEN taking the train from Rome to our ultimate destination, Naples?
Sorry, it still doesn't sound clear...

Others will chime in, but the standard advice seems to be that the train station from Siena is not convenient.

You can forget about taking the train there, I think.

There's a Eurostar train between Rome and Florence (about 90 minutes, I think). The express bus between Siena and Florence takes 75 minutes.

I can't say anything about renting a car.

The train ride between Rome and Naples should be around 2 hours.

IMHO 2 days in Rome is way too little.Am I counting correctly that you want to stay in Rome, Florence and Siena in a 5 day (nights?) period?
To answer your questions:

Training to Florence is a quick easy trip.
Bus to Siena (if it's a must)and this will take about 1 1/2 hours (both ways).
Florence to Naples is about 3 1/2 hours (Eurostar).
Doables:
You could make Florence a day trip and stay in Siena area with day trips.
Why not split the time between Rome and Florence and train from Florence to Milan?
Or-I hate to say this, but I'd skip Rome if you are that interested in Tuscany.
What I don't think is "doable" is to have one day that looks like this:
Renting a car in Florence visit SanGim ,drive and park in Siena, drive to Rome , return car and then take a train to Naples.I'd define that as very undoable.

How much time do you have in Italy?
What time of year?
Where do you arrive and depart Italy.

These are the basics. All else is open.

June 8-19th. I'd skip Rome but our traveling compaions have never been. Love it but we were just there twice. The plans are for first 2 days in Rome for whirlwind tour of city for them. Then Florence and Tuscany for next few days. We have reservations in Positano June 14-18. That is the real reason for the trip for me. So, June 11-14th, the days between Rome and Positano....How to do Florence and Tuscany? I found a really good deal on a B&B in Florence so we may just take the bus from Florence to SG and Siena and spend the night in Siena and bus back to Florence, then train to Naples. So, I think I've finally figured it all out. Any recommendations for hotel in Siena?