EuroCity, InterCity, and international trains: Reservations are required, and cost about $10 if you have a rail pass. Regional trains: Most trains that aren't on the main lines, such as connections to Assisi, as well as most Florence–Pisa–Cinque Terre service, don't require (or even offer) reservations.
We highly recommend that you book out as far in advance as possible, especially for popular and high-speed routes between major cities, such as Rome to Milan, Bologna to Milan, Naples to Rome, or Florence to Rome.
Italiarail shows a whole day's trains in the search results and can book up to 20 people at a time. Trenitalia's own website only shows a couple of hours-worth of trains at a time and can only book up to 5 people at a time.
Traveling by train in Italy means always traveling with carry-on luggage. You'll carry on board everything you have with you. It's your responsibility to bring your bags on board and stow them properly, which is a great incentive to packing light. There's no fee to bring luggage on the train.
Like I mentioned above, Italo is usually cheaper than Trenitalia but because the cabins on lower-class tickets aren't the most comfortable, I'll usually consider Italo only for trains that are less than 2 or 3 hours long unless the Comfort ticket class is cheaper than Trenitalia.
First-class seating is more spacious, with fewer seats per compartment, and usually more quiet. There is more room for luggage and it will generally be less crowded as most passengers typically travel in second class.
Some trains require seat reservations, and on other trains it's optional. If you're riding a train on which reservations are only recommended or completely optional and you don't have a reservation, you can sit in any available seat in the appropriate class you have booked.
All high speed trains in Italy require to have a seat reserved for a Rail Pass or a ticket/e-ticket with its seat assigned. In Regional trains passengers travel without seat reservations, that means there's no assigned seating.
For the cheapest high-speed tickets in Italy, go for Italo, and buy your tickets in advance. If you're on a super budget, go with the regional trains on Trenitalia. If you've never been to Italy and aren't super great at navigating confusing websites with translations that might be off, book with ItaliaRail.
Trainline lets you choose your exact seats when booking Italian train tickets, while Omio only lets you choose a preference. (Window, middle seat, or solo seat.) They charge the same fee (roughly $2 per seat) that the train companies charge when you book seats with them directly.
If you plan to take a high speed train in Italy, it may well belong to Trenitalia. Red Arrow (Frecciarossa) trains are the fastest and all passengers benefit from amenities like charging sockets, free WiFi, air conditioning and a food carriage.
They serve many popular cities such as Milan, Florence, Rome, and Naples. The seats are very comfortable with lots of leg room and your luggage stays with you so there are no lost bags. Travelers can bring food on the train in addition to a cafe car or a food/beverage cart that comes through the train.
As a general rule, it's a good idea to arrive 20-30 minutes before your train's scheduled departure time so you can get familiar with the station, find the information monitors as well as find your platform without being in a rush.
Because these train trips are all about the view, cars with wrap-around windows are common. Perhaps the most popular scenic train route in Italy is the Bernina Express, which connects Tirano in Italy to St. Moritz in the Swiss Alps.
There is typically one bathroom per train car on Italian trains. On newer trains, and especially high-speed trains, they are more spacious than what you might find on an airplane and offer the same basic features as those of a plane lavatory - a flushable toilet, sink with running water, soap, and paper towels.