We arrive in Rome at 11 am and have only carry on luggage and need to take the train to Termini and leave for Naples at 2 pm. Do we have enough time to make that schedule? We have already purchased our train tickets to Naples online.
You should have plenty of time/just don't forget to validate your train ticket before boarding train to Naples or you can have a heavy fine/you validate in a yellow machine just at the beginning of the track
You should be OK if everything is on time. From the time the plane lands count on 30 min to unload and get to the station. The train, I believe, is 20 and 50 each hour and about 30 min to Termini. Buy tickets at the window or the Tobacco shop across from the ticket windows. THOSE TICKETS ABSOLUTELY MUST BE VALIDATED. Depending on your timing you should be in Termini no later than 12:30 to 1 PM. The Leonard Express arrives on the far right side (as you face the tracks with your back to the front) and it is long walk to the front of the platform. You will need to find a departure board for the platform number for your train. Don't be concerned if you do not see it right away. The platform number/track number is often not posted until 30 mins or little later.
IF your ticket is train specific -- date, time, car number with seat reservation -- you need not validate it because is only good for that train at that time. However, some of us follow the practice of validating everything just to be sure. IF by some chance you miss that train you have 90 mins to change the ticket unless it is restricted which is becoming more common.
If you're running very late, there's an official Italian railway ticket office at the platform where the Leonardo Express departs. You might stop there and rebook, as it will be less hassle than doing it at Termini and you can buy your Leonardo Express tickets at the same time.