Deutsche Bahn requires travellers with online tickets to select some form of identification. For non-Europeans, the only practical option seems to be a credit card, and the website asks for the name and card number. Is a MasterCard debit card without the cardholder's name printed on the card acceptable?
I wrote an email to DB asking about this, and they responded,
As far as I understand, even though tickets are not transferable, the DB is not mainly concerned about ID. A credit card usually has no photo and the conductor does not need to ask for another ID to match you to your credit card. Rather, it's used to make sure that only one person can use a given ticket at the same time.
The logic seems to be that the card whose number you entered when booking is a token that is unique in the whole world, thus precluding anyone else than the card holder from using another print-out of the same ticket. This token verification process does not rely on photographs or identification of the person carrying the ticket and could therefore be quicker or even be fully automatized.
In fact, bahn.de stresses that you should take the exact card used to book the ticket, even if it's not valid anymore. Therefore, it shouldn't matter if your name is on it or not, it's the card number that matters.
At the same time, I have no actual evidence for this and never tried using a non-photo ID myself. In practice, I have never seen a conductor match the numbers but I don't travel very often with such a ticket. All I can say is that some of the them do look at the name and could be surprised.
On the other hand, I came across a forum post from someone who had no problem using a ticket on which he entered the wrong number (which implies that the conductors did not scan/check the numbers but only looked at the name) but almost got fined for it later in the trip (so another one did check!).