Later this year we're flying into FCO from the US and will be taking a train to Florence. When looking at schedules online we would take a train from the airport to Termini and then a high speed to Florence. Schedule shows only a 15 minute change time at Termini. That seems like it won't be enough time as we will have baggage. Would it be better to purchase separate tickets - 1 from airport to Termini and 2nd Termini to Florence and give ourselves 30-40 minutes to changeover? Also, does customs take long? I know they are slowly phasing in ETIAS.
15 minutes is plenty of time to change trains as long as you a) have some idea of
the layout of the station or can read a display screen b) can manage your luggage.
Once you get into Termini I don't think there is any additional screening, so you
just need to get on the train in time. Often the track for the train is not posted
until 10-15 minutes before departure anyways.
However, the situation with customs at Fiumicino is another question. There is
just no way to know how crowded the line will be, or, whether ETIAS will make a
difference or not (positive or negative).
I would not purchase 2 separate tickets. The potential issue is the delay getting out
of the airport, not the time to connect in Termini. Also, if you have both trains on one
ticket and you are delayed because of the train from FCO to Termini, you should be
able to board the next train to Florence (good news, there are plenty of trains from
Rome to Florence).
Someone else can comment on pricing of tickets on Trenitalia as the departure
date gets closer, but my thought would be to pad the train time decently, and if
you have time to kill, have a gelato and/of coffee while you wait.
You can buy separately if you want extra time.
However buying in advance is risky. If your flight is delayed or canceled you lose everything, as there are no refunds or changes possible if you arrive after the train you booked has departed. If you buy a discounted ticket on SuperEconomy fare you have no refunds or changes even if you arrive before the departure. The Economy fare is changeable only if you arrive before the train has departed.
I always buy on the spot, when I arrive, and pay the full price.
ETIAS is not going to happen before Oct 1st, at the earliest.
Perhaps you are thinking of EES?
'How long it will take to clear immigration' is in the same category as 'when is the best time to buy air tickets'.. It depends on unpredictable factors.
Buying train tickets after you have cleared immigration/passport control may not result in the cheapest ticket, but is the safest thing to do to avoid buying a 2nd tickets because the plane was late.
The price of Base fare (full fare) I think is 55€ on the same day, or 50€ if purchased at least one day in advance. Obviously discounted fares (Supereconomy or Economy) are generally all sold out a week or more before the date of travel.
The above fare is for the FrecciaRossa from Termini to Florence. To that you need to add 14€ for the Leonardo Express from FCO to Termini. That one is fixed and doesn’t change, since it’s a regionale.