You can take that “EU website” broad, as in: any website in the EU.
I just tried to order train tickets in Portugal. Not only do they ask for my passport number, they also ask for your tax id. I will, of course, totally give them my SSN for a train ride… It’s a mandatory field too. People on Reddit say you can just fill in anything, it doesn’t matter, but I hate that kind of uncertainty.
Better get used to that. Nearly every time you buy something in Portugal they'll ask you for your tax id so that they can print it on the receipt. This is because this is something you can use for some tax breaks later and also there is some lottery going on I think? Though obviously the original reason for it to exist is to combat tax fraud.
This is also entirely optional for those kind of purchases, and meaningless if you don't have one (which you probably aren't going to if you are just a tourist passing through the country) so you should just not fill that field. I seriously doubt it is mandatory.
Just go to the Alfa Pendular website and you'll see that the field is marked as mandatory, even when you select US as country of residence. I kind of expect this kind of friction when I book or purchase things in Europe.
I just tried to order train tickets in Portugal. Not only do they ask for my passport number, they also ask for your tax id. I will, of course, totally give them my SSN for a train ride… It’s a mandatory field too. People on Reddit say you can just fill in anything, it doesn’t matter, but I hate that kind of uncertainty.