Agree with BDEMARIA. I've tried to book from the US on several occasions, and never gotten all the way through for various reasons, including the CC being declined, but also just do to the notorious bugginess of this website - freezing halfway through the process etc.

Depending on your itin, you can usually just buy a ticket at the station, but some popular routes - like Rome-Florence - can be difficult at the last minute, which is why I was trying to reserve.