You can take any train of Trenitalia from Rome to Naples but note that the IC and Highspeed Services need a reservation on top of the pass. Most trains to Naples start/Stop in Roma Termini (one of the major stations in Rome)

I am sorry... what is the meaning of IC?

IC means InterCity a fast train :) 

In General there 3 major kinds of trains with even more detailed categories 

High Speed Trains (Like ICE in Germany , TGV in France) 

Fasttrains like (InterCity, EuroCity) 

And local trains 

Alright. I got you. 

Thank you. 

Travel to Naples from Rome is usually done from Roma Termini in the centre of the city, or from Roma Tiburtina (to the east; 4 stops on Metro line . Most AV (high speed) go from Termini.

Trains are classified according to speed:

1. Alta-Velocita (High Speed) and are marketed as Frecciarossa, these attract a supplement, these can be bought at the little Frecciarossa pop up booths on the platform. Show them your interrail. (1st class gets free sweets and a drink). Journey time 70 minutes, the supplement ranges from 10-14 Euros and is compulsory.
2. Also, Alta-Velocita are the ITA trains (Italotreno), these high speed services are ran by a private operator and are not covered by your pass. 
3. IC (intercity), these take about 2 hours and many go from Tiburtina only, these are covered by your pass, you may get a seat reservation for a small fee (used to be 3 Euro).
4. Regional trains (RE), these trains are not always air-conditioned, take just under three hours and need no reservation. The seats are smaller and luggage space is limited.

Arrival in Napoli is generally at Centrale station in the centre, I hope this helps.

Helena

Wow! I helped a lot. 

Thank you.

Kudos for your extensive reply! Many people don't know that Intercity trains don't require a reservation in Italy. It's very convenient actually, because this way you can travel all main destinations within Italy without a reservation. 

Yes Mattheas, I use IC trains because of this, it offers flexibility (reservations can tie you down) and the true spirit of interrail is just to get up and go! A reservation can help if you want to sit, as IC trains are often crowded in summer.

True! For me that's the best Interrail experience! (hence my signature haha) I have found the Italian IC trains to be quite comfortable, and on some routes just as convenient as “high-speed” trains, especially in the south.