Train from Rome to Sienna and then to Florence

You could do as you say, but it won't save you any travel time, you'll have luggage to store during the day and you'll need to advise your hotel of late arrival. Here's my suggestion. Take an early train to Florence (1.5 hours). Go to your hotel and either check in or ask them store your luggage if a room isn't yet ready. Take the express bus to Siena for the day (70 min each way).

Thanks for the info. Going early to Florence is the way to go.

You don't say how long you will be in Florence, but I would assume at least 2-3 days to see it all. My advice is to add a day to Florence and go to Siens fron there. Take a Grayline bus tour to San Gimignano and Siena (55 Euros). No bother with driving or train schedules and you get to see all the scenery. Lunch in Siena is on you and you should consider one of several very good restaurants on the square instead of the "hand-picked" back room one the tour suggests to you. Same amount of time and watching the other tourists wandering around is much more entertaining. Leaves Florence about 8AM and returns at 6PM, with pick up and delivery at the Santa Maria Novella train station, which is walking distance from anywhere in Florence. Local guides at both places. Was excellent.

Hello,
We have four nights in Florence and plan to do quite a bit in four days. I know it is packed but doable. but all plans are tentative except once you make reservations for museums.
The first day we will travel early from Rome to Florence, leave luggages in the apartment if the owner let us do that, then take the bus to Sienna around noon. The next two days [Saturday and Sunday] will be in Florence for garderns, walks and museums. We will travel to Pisa for a quick peek and Cinque Terre for the last day.