There used to be train service on that ferry as well. Fond memories, in particular from the time when it was operated with the ICE-TD pseudo HSR (a diesel train, because German railroad is embarrassingly allergic to investment into any parts of the network that might be involved with crossing borders). Train goes into the ship, passengers disembark the train to stroll around the ship, just enough time passes for two cans of Tuborg out in the wind (benefits of not driving) and then it's back to the train again.