You need a reservation for all high speed trains like Frecciarossa (10€), but also for Intercity (3€) and Eurocity (IT-CH -10€in Italy, 11€ cross border). 

Regionale and Regionale Veloce trains are reservation free.

@Angelo Thanks for the fast reply! Have a nice day.

Cheers

@Rico Matzinger 

you’re welcome.

I have a tipp for you, use not the Interrail.eu reservation service for Italy (you pay more, and often you got the wrong class)

You can book online via oebb.at using the one way ticket option and at adult the discount Interrail/Eurail Globalpass. 

Or at any Ticket Desk of SBB CFF FFS, Trenitalia, DB or ÖBB. 

@Angelo

Good to know! Thank you for the help.

But this trip is very hard to make RES free-as on some parts of this route there are simply no Regionale. It all differs perroute what is possible. Can set app to ´REServ free´ to see what it then delivers, but in IT many such trains are not in system so cannot come out either.

@Angelo I thought IC services had gone back to the pre-COVID arrangement: reservation optional. Or is that not correct?

I don’t know. On the timetable they are Reservation compulsory, but the same sign is on some regional trains also. 

But tickets are sold with reservations, like precovid it could be that you have to stand because there are no reservation screen to see if a seat will remain free or not. I personally would suggest that Eurail Staff should ask directly Trenitalia and inform us about the current rule. 

I just see the general remark "Servizio di 1ª e 2ª classe prenotabile” - but that just means reservation is possible, correct?

For the Frecciarossa, it says however: "Per viaggiare con i treni ALTA VELOCITA' FRECCIAROSSA è previsto il pagamento di prezzi di mercato comprensivi dell'assegnazione del posto”

And that does imply a compulsory reservation.

In the timetables Trenitalia send to MERITS (and as such displayed by e.g. DB and ÖBB), they don't mention anything anymore about mandatory reservations (not even for the Frecciarossa) - which is erroneous IMO.

I agree that a longer IC journey without reservation is not a good idea, especially since you can't see in the train which seats have been taken.