Timing of train

If your plane is on schedule and arrives at 9:10am The Leonardo da Vinci train from FCO to Rome Termini train station takes 32 minutes. The unknown variable is how long it will take you to get through immigration and customs at FCO. I would have estimated a couple hours. If you buy a non refundable train ticket for Naples ( it departs from Rome Termini) you won’t want to miss your train. I said I would have estimated a couple hours getting through FCO. That was until friends flying there a few years back landed at an FCO that had smoke billowing from one of the buildings.
The delay the fire caused meant they didn’t get through FCO for 3.5 hours. The airport was closed for twelve hours..
That doesn’t mean the same thing could happen again- just that the vagaries of unpredictable events like strikes and emergencies do sometimes happen. Not a big chance, just a chance.
I would allow wide berth on the time to get through FCO as this is expected to be a boom year for travel to Europe and Italy is one of the top destinations. You can book discounted seats on a high-speed Frecciarosa train taking 75 minutes from Rome to Naples at Italiarail.com There is a Frecciarosa train leaving Rome Termini at 12:50pm.
If you arrive in Rome an hour before your train to Naples you can always have lunch in the Eternal City.

For your arrival day, it’s best not to buy train tickets in advance, in case your flight is delayed. For other train trips, if they’re on high-speed trains, you may be able to get bargain rates buying in advance, but then you’re locked into taking that particular train - if you miss it, there’s no refund. For the Regionale trains, there’s no point to buying in advance.

I agree with the previous reply. It's often best not to buy tickets for the high speed trains (Rome to Naples) in advance as these have compulsory seat reservations which are specific to train, date and departure time. If your flight is late arriving and you miss the trains you've booked, the tickets are worthless and you'll have to buy more. You can buy tickets for the high speed train at the airport.

There's absolutely no point in buying tickets for the Leonardo Express in advance, as they're easily purchased at the airport. Be sure to validate your tickets prior to boarding the L.E. or you'll risk hefty fines of about €50 PP which are collected on the spot!

If you'll be travelling with your smart phone, you might find this useful - https://apps.apple.com/us/app/trenitalia/id331050847 (there's a version for Android phones also).

There's also this App if you'll be travelling to other countries in Europe - https://www.thetrainline.com/information/apps .

This is a bit dated but you might find it useful - https://www.youtube.com/watch?v=6n8rITO1Eek .

Do you have a smartphone? I am asking because there is free wifi at FCO airport.

You can install Trenitalia's app at home and get used to it. Then buy a through ticket from "Fiumicino Aeroporto" to "Napoli Centrale" after the baggage claim. Assuming you are staying in Naples and not in the Amalfi Coast

If you want only protected connections, avoid third party apps like TheTrainLine

Thank you everyone for the replies. So it sounds like I need to wait until I arrive to FCO to book my tickets. I was just concerned tickets wouldn’t be available if I waited. So rather than being an issue of availability it’s more the fact that you can find cheaper tickets in advance - am I understanding that correctly?

There are such frequent trains between major destinations that you don’t need to worry about them selling out. The Regionale trains officially never sell out anyway. Bargains are a possibility if you book well in advance, but then you’re locked in - not worth it on arrival day.

Hi Lindsay, There’s plenty of trains- about seventy daily and four to six hourly on the Rome to Naples run. Regional trains take about three hours, and the Frecciarosa trains take 75 minutes. There are two or more of the Frecciarosa trains each hour. You can see the schedules at Italiarail.com for a sense of the pricing. Dynamic pricing works like airline tix- the more seats that sell, the more prices rise. Already on June 7 there are Frecciarosa tickets available for $30.
A couple weeks later on a random look the same train run is $59 rising to as much as $101.
Do what you feel comfortable with either by building in four hours or so to catch the train you’ve pre-booked, or just waiting until you’ve passed through Italian Customs and Immigration and buy them online.
Whatever you do, Italy 🇮🇹 is a fantastic place for your first European visit. Have a great Trip!

Train pricing do not work like airlines and it isn't actually dynamic, mostly because prices are capped. The maximum you can pay is the walk-up fare, called BASE on trenitalia.com and Flexi on italotreno.it.

Both Base and Flexi fares are published months in advance together with the prices of the discounted tickets. There can't be surprises on the day of travel.

Discounted tickets are capacity controlled, When Trenitalia and Italotreno have sold all of them they are gone. Nevertheless, you can't pay more than the Base/Flexi fares.

ItaliaRail is a re-seller, that's why they show prices in $ and use city names in English. Unfortunately once in Italy everything will be in Euro and you won't see many "Naples" or "Rome" on the Departures boards.