Identification card for foreigners in trains in Germany

Passports are (unusually!) not enough. From the DB site:

In order to book online tickets you need:

An identification card (BahnCard, bahn.bonus Card, credit cards*, ec-card/Maestro or ID card**) for booking via Internet and as identification during fare controls on the train.

• The ID cards of the following countries can be used as identification on the train: Germany, Austria, Switzerland, France, the Netherlands, Belgium, Luxembourg, Italy and the Czech Republic.
• Passports are not accepted as identification documents.

So book with a credit card, bring the credit card with you, and you'll be fine.

In addition to jpatokal's good answer, it should be added that there is also the possibility to buy a saver fare over the telephone.

http://www.bahn.com/i/view/USA/en/home/contact/international_service_hotline.shtml

Normally, when ordering a ticket via the phone, you receive a code that you can type into a ticket vending machine prior to departure to receive a paper ticket. No form of identification is required in this case. However, the code can only be used once, so must be kept secret and you shouldn't lose the paper ticket. The particular website linked to above however states that the travel documents will be made available for collection in ticket offices, which can be found in train stations at the major hubs. The following website however states that vending-machine pickup can be used:

http://www.bahn.de/i/view/USA/en/prices/onlineticket/how-to-buy-a-ticket.shtml

It should be noted that the phone number listed on both sites is not a standard landline number and may cost a lot to be called from abroad.

Some travel agents abroad also have a DB license and should be able to sell you a saver fare ticket.

the identification card needs to be swipeable, and passports do not meeting this criteria.

The identification card is swiped at the same time that the online ticket is scanned by the conductor, and this data is saved by DB. This helps ensure the ticket is being used by the person it was intended for.

I personally have never seen a conductor let this slide. It's not a matter of them being reasonable, it's their job to enforce these rules to guard against fraud and theft, and they take that seriously. You should too.

I thought it was quite clear in their terms and conditions. One form of ID, and the easiest for anyone who isn't German or living in Germany, is the debit or credit card that you used for paying the tickets. (They don't actually want to know who you are, they want to make sure that you don't buy cheap tickets and sell them on, and a credit or debit card works just fine for that purpose. No passports accepted / required. No ID whatsoever or names / addresses required for other people travelling on the same ticket, only ID for the purchaser).