The issues/gotchas relate to the website - which is notoriously difficult for US purchases. The chances are great that you will get to the section for purchase and then find that your credit card has been declined. This is b/c the Trenitalia website has a reputation for lax security and most US banks have a block against any purchases from the site. *If* you can successfully purchase the ticket, then you should have no problem when you get to Italy.