I'm not sure what the track planner is but it sure doesn't determine where Interrail is valid.

The one country pass is also valid on Sicily. Or on Sardinia for that matter. Since both are in Italy and Trenitalia runs trains there.

There is a section on the site that say plan your journey. SO I can plan my route out in Italy. You enter your next destination and it marks the route. So I can go from Pisa to Napes for example as I intend to visit Pompeii and Sorrento. Then I want to go from Naples to Cantania in Sicily but on this Plan your route page will not let me select in cities in sicily so it makes me think that travel to Sicily is not possible. 

How would I get from Naples to Catania in Sicily. Please can you advise me?

Interrail is just a company that sell the pass.

For Route planning you may ask a Rail Travelagency or Railticket office of the participating Railcompanies. Some of the Railcompanies even have Customer Service Hotlines.

As Interrail covers more than 30Countries with hundreds different Railcompanies it´s difficult for them to have all Informations about the trains 100% actual.

I guess you mean the Railplanner of Eurail

The railplanner doesn´t list all trains as some companies like Trenitalia (Italian Rail doesnt share their full timetable with Europe ) This means there could be on some routes trains were Interrail is valid but these doesn´t show up on the Railplanner of Eurail.

Because of that i recommend to use most times a local railplanner for the Country/Company you intend to travel on.

Welcome to the wonderful world of how all those railways in their very own burocratic ways try to stifle international travel on their means. Now you have discovered that there are places in this world where there are other ways of doing things as on that island in the North Sea. Thats one of those joys of travel-to discover other things. 

Start with browsing the country info-it gives much detail about how it works in that specific country.

Also be aware that in IT there is also a range of local railways that will not except the pass-esp. right in the area around Napoli and also the most famous line on Sicila: around the Etna. Your pass covers the extremely state-owned TrenItalia, which does serve the whole country and its 2 main islands. Its hi-speed FRecce trains need to be reserved as do the seats in the IC (by day or per notte-night)-do that at a counter.

Aslo just a hint: be aware that IT-due to its history of being first and severely hit by that covid, is now also still the one to have most stringent anti-covid measures, you have to comply with before setting off and whilst there.

From Neaples there are some Intercity trains that run to Sicily (From mainland to sicily they go on a boat with the train to get over). You need a reservation for 3€ for the train.

You can also use one of the High speed trains on the route (they are not realy faster on this route), and thannin Villa San Giovanni change for the express boat to Messina (Sicily). Highspeed train (Frecciarossa or Frecciargento) reservation 10€. Express Boat not included in Interrail, but it cost only about 5-10€

Is the intercity one the same as someone else suggest that goes from Rome to Sicily and it just goes onto the train with you in it? So I presume if I was on this train I would not have to change for the boat and then another train as if I was using the high speed train. Does this train carry on your journey to for example to Catania without having to change and does this ticket include the ferry charge or do you always have to pay extra once on the boat in the train or does the boat ticket always have to be booked separately regardless of which train one is on.

This intercity goes onto the ferry indeed, with you in it. And in that case, you only need a train ticket (your Interrail) and a reservation.

Thank you. How far does this train go to? Would I then need to book another train reservation to get to Catania?

For Italian timetables, please check www.trenitalia.com.

On the ferry you can stay in the train or exit and go at the top of the ferry to see the sea. 

Please check trenitalia.com to confirm the timetable but from Napoli Centrale should go the IC train from Rome to Siracusa via Catania at 9:50 IC723/721 (this train runs together with IC to Palermo) and at 13:45 IC 727 to Siracusa via Catania.

And the night train ICN 1955 to Siracusa via Catania at 23:24.

That is great info, thanks. It would be interesting to take a night train, not done that since travelling in Eastern Europe across Bulgaria and Czechoslovakia back in the 80s before the fall of communism.

How do the night trains work this days. Are there sleepers with seperate compartments that you share with a stranger or does the whole carriage just convert i sleeper? Its so long since I have done this long travels on trains.

This community is wonderful , thanks for all the replies it really helps. No doubt I will be posting more questions.

Night trains are more modern but basically work the same. Except that in Italy, you still cannot book a single bed or berth for yourself in a compartment because of COVID. You still have to book a complete compartment.

I bet that would be expensive. What would be the alternative to getting an compartment, just curling up on a seat in normal carriage?

Correct. And advance fares (check on www.trenitalia.com) are often lower than Interrail supplements.

What do you mean Interrail supplements? You mean there are also supplements to pay on some trains?

I mean Interrail reservations, specifically for night trains here.

I see, is it (www.trenitalia.com) that run that train from Rome to Syracuse that I could pick up in Naples? And is the night train same company?

Yes it is (2x).