Firenze Santa Maria Novella to Siena by taxi

If you go to rome2rio.com it gives 2 taxi company phone numbers. it also indicates it's an hour trip and around $120. Don't know how feasible it would be as the cab would of course make a return trip to Florence taking up at least another hour. All you can do is call and ask.

I doubt $120 are enough. I think €180 is closer to reality.
Regardless. Since they are arriving at Florence by train, why not take another train to Siena from Florence SMN? There is a direct train (no change) every hour at 10 min past the hour, and the travel time is only about 1h30, not much more than by car (1h 15m with no traffic) and maybe less if they travel during rush hour traffic. The walk from one platform to the other to make the train change is certainly less than the walk from the arrival train to the taxi stand. The train cost is only about 10€ per person. The bus takes on average 15 min less than the train, but the walk from the station to the bus station is about 10 min, so in the end it’s about the same time. I would continue by train unless they are in a rush to get to Siena that even an extra minute counts.

Thank you, Roberto! If there's a direct train, hourly, that sounds like a good plan. I'll tell them.

They can buy the ticket from Florence to Siena from whichever station they depart from, for example if departing from Rome they can purchase it in Rome, or they can also utilize the Trenitalia app, that way they don't need to stop to purchase tickets in Florence, in case they have a short layover in Florence.

If they purchase a paper ticket (at the ticket office or the automatic kiosk), they must validate (date stamp) the ticket at one of the machines on the platform before boarding. The Flirenze-Siena train is a Regionale commuter, and works like MARTA in Atlanta (no reserved seats, first come first served) but the ticked must be stamped before boarding. If purchased on line or via App that is not necessary.

Once at the Siena station the taxi stand is in front of the station, see those white taxicabs lined up on the right of this image.

Thank you again, Roberto. You added details I would probably have forgotten to tell them. Did not think about buying the ticket on the app.

Ruth,

Keep in mind that (digital) Regional train tickets booked online, via Trenitalia’s website or on its app, must now be validated by checking-in (on the website or app) prior to boarding the train.

Read all details here: www.trenitalia.com/en/purchase/digital-regional-ticket.html