I am purchasing a France-Italy railpass for my trip this summer (June/July) and I know that I have to make reservations for my train travel in Italy, but I am not sure how to do this. Do I need to make reservations for trains during the day, or just night-trains? Should I reserve way in advance, or can I wait until I arrive in one city, say Rome, and reserve a seat on a train to Naples for 4 days later? Can I reserve online? Also, I am traveling from Marseilles to the Cinque Terre, so do I need to reserve a seat on that train too? I know I am asking a lot of questions, so thanks very much in advance for everyone's help!
A properly validated pass (assuming Eurail 2-country 1st Class) will cover your base train fare. However, it does not cover seat reservation fees. Unless you want to reserve seats or couchettes on night train, you don't have to reserve in the U.S. in advance. As soon as you can after arrival, you go to the train station, show them your pass and tell them which train you want to which destination and on which date. The agent will process everything including the seat reservation.
As a side note, most trains that you will be traveling on will require a seat reservation. Only the regional locals do not need them. For those trains, you just hop on and take a seat. Many runs (i.e. Marseille/CT) will require train changes. If you get the right one, the journey takes just under 8.0hrs with 2 train changes. You will get one ticket for each of the 3 legs. All three legs are high-speed and require reservations. The agent will do all of this at the same time.