Buying a ticket with Trenitalia online is a gamble. I bought tickets many times with my US credit cards, Am-Ex as well as MC, but I was denied my card just as many times. Most maddening is the fact that they block your your user after a couple of attempts, and you can only unblock it calling from within Italy, where the call to the call center is quite expensive, and the unblocking is not even immediate.
The acceptance of US issued credit cards in automatic ticket machines at railway stations is also random, which means that you might have to prepare for a long line at the counter.