Trenitalia asks what carriage number I want and which seat number. (The window/aisle choice I understand.) I can't find a picture of the inside of a train and have no idea how many carriages/cars are on the ES. My concern is that we need to face in the direction that the train is going (motion sickness). Also, can I do a round trip or do I have to book Rome to Florence and then hope we can get back on the same day? Thanks for info.
That function is only when you want to sit next to someone who already has a ticket. Otherwise, you just choose aisle or window.
(continued) Yes, you will have to book again to get back from Florence to Rome. Have you already registered and received a password from Trenitalia? If not, registering while you're purchasing tickets has proven problematic. Try registering first and awaiting the password, then purchasing. There have been reports from many Americans that American credit cards cannot be used. If not, you can purchase the tickets immediately you arrive in Rome. Termini station has many self-serve kiosks that display in English, and there are helpful people around to assist you. As for the seats themselves....many of the faster trains have the vast majority of seats facing forwards. If you book on-line, you'll have to take your chances, but they're pretty good odds.