You can check timetables with an general timetable website like german railways bahn.com or via italian railways trenitalia.com, for only using Interrail in Italy you can buy an Interrail One Country Italy. Tickets for regional trains are very cheap in Italy. You can also buy an Travel Pass for Trenitalia Regional Trains “Promo Italia in Tour - Trenitalia” for 3 consecutive days 29€, 5 consecutive days 49€, but trains of Trenord (to Lake Como) are not included. All other regional trains operated by Trenitalia from Milan to Cinque Terre, Pisa, Florence, Siena and Rome are perfectly to get there with regional trains. From Rome to Neaples, there are regional trains, but they need way longer than high speed trains Frecciarossa. With Interrail you need a reservation (10€ via ÖBB - tickets.oebb.at - in the link how it works to get reservations

or at any Trenitalia counter in Italy or France or with an additional booking fee via interrail.eu). You can also book an normal Ticket for this leg. 

To Sorrento you should get there with regional trains (but I’m not sure if this is a Trenitalia service), but to Pompei only a local train (not part of Interrail) gets there, but it’s cheap. 

P.S. Malpensa Express from Milan Malpensa Airport to Milan, Interrail not valid. Trenitalia Promo in Tour not valid. 

Check timetables on www.trenitalia.com. Do not use the rail planner app to plan. It's not reliable and up-to-date enough for that purpose. You can also check the normal fares and see if a pass is really cheaper (taking mandatory reservations for long distance trains into account).

You can buy reservations at the ticket office or via ÖBB using these steps. Both are cheaper than booking via Interrail, since they don't have booking fees.

Thank you Angelo, the regional train pass sounds good but I am thinking if it is consecutive days travel then it wouldn’t really work would it for me as I am staying a few nights in each place. Can you use the interrail pass on regional trains or is that false economy?

Interrail Pass is valid on regional trains (also Trenord regional trains to Lake Como), without reservations. You need your Interrail Pass (mobile or paper), add the trip to the pass and hop on the train. 

Blunt but short: do not use IR for this-unless you are definite into using these superfast Frecce hi-speed trains as much as poss. Just buy when you go-there may evne be a small senior discount. But yes-the cheap pass Angelo mentions is only for consecutive-so it depends on how long you spend to stay if it pays off or not. Around the regions of Roma and Napoli-and in Umbria-there are also quite cheap local regional (day)passes, incl also bus+metro etc.

Buy advance-and do that now-for the overnight Nap-Sicilia-note that for sleeper etc there are strict covid-rules and they only sell whole coupe. DO be aware that IT still has the most stringent covid-rules of all EUR- a green pass is indispensable. Get informed how your NHS vaccinations can be turned into such a thing

HI, interesting, I wonder if there are many super fast trains on the route I am planning, not 10 percent certain. Milan> Lake Como (Verenna)> Cinque Terra> Florence> Sienna > Rome > Naples (then my Sicily leg), so whether IR would be the best option or not? 

Couple of quick question too: if going to Cinque Terra from Lake Como I guess I would have to go back to Milan Centra first and then get another train (would this be booking two different trains on this route or one straight through) and actually I wondered which would be the best route, to get a train to Pisa to them get that local train to Cinque Terra or go to Florence first, then Pisa. Do you think which is the best route to do get there?

Regarding the express from Naples to Sicily, which is the best site for me to book that train now as need to do it as you say in advance as soon as possible.

Thanks for your help

Do you mean the day train or the night train?

I have not decided yet whether to take the day or night train. Do you not book either the day or night train on the site like www.Trenitalia.com site.

I was looking about the tour Italia 3 or 5 day passes but you have to use them every day so not good for me, once you don’t use them apparently they become invalid. 

I was then looking at the Trenitalia Pass which you can get for like 15 days but apparently they are not valid on the Regional trains, and I think that some of the trains that I will be taking will in fact be regional like the one from Lake Como (Verrenia) to Cinque Terra (is a regional train. 

So the two passes are not really any good so it would be back down to using Interrail which is 306 euros for 5 days, or I could probably buy one for just fours because some of my trips are surely just regional trains and may well be cheaper than using a travel day. I mean for my itinary which I already mentioned on here, I wonder which ones I would actually need a IR pass for. 

This is my route:

Lake Como to Cinque Terre

CT to Pisa

Pisa to Florence

Florence to Sienna

Florence to Rome

Rome to Naples

Naples to Siracuse (Sicily) - Day or night boat train.

How many IR travel days do you think would be neccesary for such a trip?

Thanks

The best really is to check prices on www.trenitalia.com and then see if you can cover some of those days with a pass (including reservations if needed).

As to night trains in Italy: you can currently still book only a complete sleeper or couchette compartment, also if you're alone. You cannot book yet a separate berth or bed. The normal fares however are often lower than the Interrail supplement. Nobody knows when this will change and when you can book a single berth/bed again.