Dobry den. Just as in the rest of IT: those by TrenItalia.

I have been there a few yrs ago and I must add that the network is quite limited and not in very best condition. Delays and cancellations are/were common - but that may have changed or perhaps was seasonal. It is mostly a north-south line from Cagliari to the ports where the ferries to mainland go plus an odd side-line branch.

Beside TrIt there is supposed to be a narrow gauge local comp SFS=FerrovieSarde, but I think that their services are now limited to some steam/touristy seasonal outings. Takes no passes.

In fact, if you want to buy a pass for mostly there, I would say: think again, buy/borrow a good guidebook about also transportaitons. Most common used is the BUS-also longer distance, much better fit to how little the demand is and also much faster as they can use better roads.

BTW: Tr-IT ALSO offers their own 3 and 5 day passes, ONLY valid in regionale (all trains they run there, there is no IC or Frecce), these cost 29/49€ and thus are a lot cheaper as IR-pass. As I know quite a few Cesky seem very much inclined to use the vlaky. It is called TourITalia and can be bought as paper ticket at all ticket outlets (or should be).

You should also know that locals do not really speak IT among them-its more like a Spanish dialect similar to Mallorquin.