Is this seat reserved?
pre-no-TA-to — stress on third syllable. 'Posto' is POS-to.
When boarding a high-speed train and finding someone in what appears to be your seat, or when you want to sit in an unoccupied seat.
'Prenotato' (reserved) is the past participle of 'prenotare' (to book/reserve). On Italian high-speed trains, every seat must be reserved — there are no open seating carriages. Seat reservations are printed on the ticket. On regional trains, there are no reservations and seating is open.
Credo che lei sia al mio posto.
I think you are in my seat.
Polite way to reclaim your reserved seat
Il mio biglietto dice posto 45. Dove si trova?
My ticket says seat 45. Where is it?
Ask for help finding your specific numbered seat
Posso sedermi qui? Il posto non sembra prenotato.
Can I sit here? The seat doesn't seem to be reserved.
Ask before assuming a free seat is available
On Trenitalia's high-speed trains (Frecciarossa, Frecciargento) and Italo, all seats are reserved — the seat assignment is on your ticket. On regional trains (Regionali, Regionali Veloci), seating is open and first come, first served. Intercity trains have both reserved and open seating sections.