Troubles with straightforward Visa/MC Debit cards are widespread in the USA, as their perception of debit cards is different to the rest of the world, and they are seemingly only given to those who are poor credit risks for the equivalent credit cards. For example, if you reserve a rental car from the UK using one, it is fine, but if you try and do a new booking at the counter there I have been turned away for offering a Visa debit, not credit, card.
The other issue is websites which insist on validating a US Zip code in the address, where the website developer has not envisaged not having one. It would be unlikely that Amtrak would have fallen for this but it has certainly tripped me up elsewhere, and doesn't error at the logical place.