Train reservations, are they necessary?

Depends on what you mean by necessary. All trains in Italy exception for the regional (slowest) require reservation. Therefore, it is necessary to make a reservation. If you are asking if it is necessary to make advance or early reservation. The answer is NO. Train rarely if ever sell out so you can buy a ticket with the reservation within 30 mins or so of departure. But it is not a bad practice to make your reservation a day or two before.

Sue - We usually buy our tickets just a day or two in advance once we get to Italy, but if we have a firm schedule, we try to book in advance to get discounts. It just depends on how flexible you are. At the beginning of April, the trains we took from Rome to Venice, Venice to Florence and Florence to Rome were completely sold out in 2nd class. I don't know at what point they sold out because we purchased in advance to get the Mini fares. At that time there was also a one day transportation strike but the National trains were running. The trains were completely sold out all day on the Venice to Florence route and most of the National routes. The only time we tried to by a last minute ticket for one of the National routes (not local slow trains), was from Naples to Rome. We couldn't buy tickets even 1 hour in advance as they were sold out. We did end up getting on a slower train a couple of hours later. This was in September a couple of years ago.

Sue, the train runs you describe are great Italian high-speed runs. There are frequent trains during the day. Normally, you can buy your tickets in Italy a couple of days in advance. All of the high-speeds will require seat reservations which come automatically when you purchase the tickets. Purchasing these tickets will lock you in to a specific train departing at a specific time on a specific date. This should not be a problem if you plan well. With Trenitalia permitting U.S. credit cards for the purchase of train tickets (they were blocked for a couple of years), you can easily go online and take advantage of the mini-fares. Great discounts on the high-speeds. These mini-fares are often gone if you purchase tickets a couple of days in advance.

Sue, You might have a look at the Trenitalia website, as that will indicate which trains require compulsory reservations. If you're buying P-P tickets, the reservations will be included in the cost. You'll be assigned a Car No. (Carrozza) and a Seat No. (Posti). On some routes, there may be both Regionale trains (no reservation required) as well as fast trains (reservation compulsory), so you'll have to check the website to determine which train is most preferable in terms of cost and travel times. I haven't checked each of the routes, but it's almost certain that Venice - Florence and Naples - Rome will be EuroStar Italia / Alta Velocita, which will require reservations. You might have a look at This Excellent Website for information on rail travel in Italy. Happy travels!

I'm not sure that it was spelled out for Sue, if you wish to ride anything but the slowest trains in Italy, i.e., if you want fast trains or fastest trains, that's IC (Inter City) type trains or better, you will need compulsory reservations. The difference is this - if you buy point to point tickets on your trip those reservations will be part of the price of the ticket, mini, full price, its in there. If you are traveling on a pass, you need to budget money to buy those reservations. I haven't done it myself because I don't use those passes, but I have heard here that they are about €10 per person per segment, each way. What you show in your OP is 4 segments, but you are missing the Florence to Rome/Naples sections.