Roma Termini Booking

Where are you going?

There is no way to predict how long your arrival will take
Flights can be and are often delayed, lines at Immigration etc

Best advice is to wait til you arrive
Get the app and purchase tickets once you are through Immigration

There are multiple fast trains every hour and all fast trains require seat reservations
The Leonardo Express does not have seat assignments

If you feel you must purchase in advance make sure you book a refundable/changeable ticket (likely same cost as just waiting til arrival)
Read the restrictions

I suppose 3 hours past expected arrival time would be sensible (referring to a train from Termini here)
If you find yourself with extra time grab a bite to eat at the station or a picnic to take on train

1. The LeonaRdo Express is a typical airport train: no discounts, no reserved seating. It can't sell out. No reason to risk 14 € getting tickets in advance.

2. Assuming you are taking an high speed train from Termini, there are only reserved seats both on Trenitalia's and Italotreno's bullet trains. Passengers who are not on a budget get thorough tickets either at FCO station or via app, at the baggage claim area. If it was a mistake, they wouldn't.

Otoh, some get discounted tickets in advance because the price gap with the walk-up "BASE" fares can be relevant for families.

You can easily check by yourself: make a search on trenitalia.com and look for trains departing today, within 40 minutes. Anything sold out?

Two years ago I did what you are planning, although you didn’t say your destination, pre-booking TrenItalia ticket. I landed FCO with carry-on. After exiting through immigration, took Leonardo Express to Termini to catch train to Venice. Looking at my old timing notes, I booked business class silenzio.

Using flight history, I allowed for average flight delay, and booked the train for four hours later. I could have booked three hours but if felt too rushed and I wanted “just in case” timing. I chose silenzio because it not a walk through car, safer if I dosed off from jet lag.

If anyone is questioning why I didn’t fly into Venice, $600 difference in airfare was a huge incentive. Even with the higher train fare I saved $500 RT. And what’s four hours out of a monthlong trip.

I picked this up from one of your previous posts:

Hi. I’ll be traveling from a Rome to Salerno via high speed train.
Either Frecciarossa or Italo. I’m unsure if I can purchase an advance
ticket since I’m unsure of my timing getting from the airport to the
main station.

So is this still the plan? FCO to Roma Termini to Salerno?

As above, there's no way to predict how much time you need: your plane may be on time, it may be late, it may even cancel (although I hope not!) Amount of time to get your bags is another unknown: we waited well over an hour at FCO once. There's absolutely no need to pre-purchase a Leonardo Express ticket: as noted above, it doesn't sell out, and there are no seat assignments so purchase it when you arrive.

You didn't ask about Termini to Salerno. Was that supposed to be part of the question?

As noted above, you can't reserve seats on the LE to Termini. IF you are asking about booking a reserved seat on a train from Rome to Salerno, note that tickets on the Freccia and Intercity trains (Trenitalia) and Italo trains automatically have a reserved seat. What type of seat preference do you want? If you chose to take regional trains (which take twice as long and require a change) then you would not be able to reserve a seat.

I would not recommend booking in advance for an arrival day with an international flight, unless you book something that is changeable up to the last minute. There is no way to predict if your flight will arrive on time, how long it will take you to navigate immigration / EES, fetch your luggage, etc.
When planning our trips, for any train travel after arrival day, we book those in advance; any train travel on arrival day is booked in route to the train station. Last fall we had friends that purchased train tickets in advance (on arrival day) from Roma Termini to Bologna allowing what they thought was an adequate buffer; it was not, so they had to purchase new tickets at the station.
My suggestion is to download both the Italo and Trenitalia apps and set up your accounts. Then once you are on the LE to Termini, purchase the cheapest ticket that works best for the timetable you have at that moment.

Given that your trip is a bit over a week away I doubt pre-purchasing your ticket will be a much of a saving over buying when you are there.

Save yourself the worry of the flight being on time, and just buy when you are there

Trenitalia (official train company) has direct fast trains to Salerno 11, 12, 1 it is a 2h 6m journey
Also has direct IC trains at 11:26, 12:26- journey is 2h 58m- IC trains are similar to fast trains with reserved seats- just makes more stops
(Direct does not mean Non Stop)
https://www.trenitalia.com/en.html

ItaloTreno (private train company) has direct fast trains at 10:40 and 1:40- 2h 12 m journey
https://www.italotreno.com/en

Download the apps for both, set up your accounts and familiarize yourself with it

Once you are through Immigration/EES and have your baggage you can use the app to book your tickets. Or you can purchase at a kiosk or window at the station in the airport.

Search for

Fiumicino Aeroporto to Salerno

That will include your Leonardo Express leg. You will want maybe 30 min maximum between that and the fast train from Roma Termini. If you need to grab a bite give yourself more time. I suppose you could book each leg separately if you must

The Leonardo Express runs every 15 min- no reserved seats- 32 min journey to Termini

I'd guess you could be on the 12 or 12:26 Trenitalia train- it is possible you could make the 11:26 but I would not expect any earlier than that.

Wait til you get there or buy REFUNDABLE/EXCHANGEABLE tickets. You will have to read the restrictions and rules and know what to do to change your ticket. (Honestly I would not want to deal with that on a jet lag arrival day)

Just buy tickets when you get there. There are plenty of seats available- Base Rate is 53euros- which is what you would end up paying if you purchased a discounted ticket now that you end up not being able to use.