I thought I'd spread the message about my recent successful advance purchase of Trenitalia tickets using Citibank. The trick here is you need to have a Citibank Mastercard and with that, create a Citibank MC Virtual Number Account. Refer to this Citi webpage on how to create a Virtual Account :-
Use the virtual MC credit card number, 3-digit CVC# and expiration date as if you were using your regular Citibank Mastercard. It might not work the first time when you try using the virtual Mastercard on the Trenitalia website. I tried a second time 10 minutes later and it worked. Using this method, I didn't need to ring up the Citibank call center to unlock or unbar my regular Citibank Mastercard for the Euro currency transaction.
** Note : I can only relate to you my experience with the Citibank Mastercard. I don't own a Citibank Visa card. So I don't know if Citi Visa cards would work using this method.