Rome airport to train station

No way to predict
Just buy your onward train ticket when you arrive
There are multiple trains every hour

Book with official train company
https://www.trenitalia.com/en.html

Or get their easy to use app

Any suggestions for the earliest departure from Roma Termini station?

To where?

We are going to Florence

I have been looking at https://www.trenitalia.com/en.html. Thanks for the tips on buying tickets then.

There are multiple trains from Roma Termini to Firenze SM Novella

As posted above-purchase your ticket once you arrive either on the app, at a kiosk or at the counter

Thank you for the tip. I will get the app.

https://www.italotreno.com/en/destinations-timetable/rome-florence-tickets

Italo also runs trains on that route. When you arrive check their schedule and fares to see if they would be a better option.

ItaloTreno also has an app

Just be aware that the train from FCO to Termini -Leonardo Express-is operated by Trenitalia

( don’t know if ItaloTreno sells the LE)

Buy tickets at the airport station when you arrive at the full price (69€ in standard class)
Purchasing in advance, although might give you a chance for a discount of up to 35€, it is not worth the risk. If your flight is delayed for whatever reason (weather or mechanical) and you miss the specific train you book, your ticket will be entirely worthless.