Passaportes são (raramente!) não são suficientes . Do site do BD :
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.
Portanto, reserve com um cartão de crédito, leve o cartão de crédito com você e você estará bem.