If I prebook train tickets for Italy to go between Naples-Rome, Rome-Florence, Florence-Venice...are these "use or lose". If we were to arrive late for the train, would the tickets allow us to board a later train...or would we have to buy new ones.

Trying to determine whether to prebuy tickets or wait until we arrive at the train station.

We're not planning on being late necessarily but who knows how we might get turned around in a strange city.

How crowded are the trains usually..AV especially? End of June/start of July timeframe. Is it easy or impossible to get tickets at the last minute?

Any other pros/cons to prebooking vs. buy at the station.

In general, the tickets for faster trains, which come with seat reservations, are good for that train only. You would have to rebook to get on another train and pay a penalty.

This came from one of ellenem posts too:

Everything you ever wanted to know about the Italian train system:

http://www.slowtrav.com/italy/trains/index.htm

All those tickets will be easily available IME in Italy - and you can do it all at the airport train station or rail desk upon arrival if you wish or at any station

the automatic ticketing machines in stations are idiot proof (i used them) and it's all in English and U.S. credit cards work in them so you need not wait in what can be very long lines at ticket windows

www.trenitalia.com is the Italian rail web site where you can buy tickets, including at about 20% discount or more but this site is nearly impossible for many Fodorites to use so i'd just wait until Italy and then buy them all at once.

You can make a booking change at no charge up until three hours after the original train's departure. See http://tinyurl.com/5hgdgy.