One such liner, Cap Arcona, designed more for luxury and volume than for speed, entered service, primarily for the South Atlantic trade, in the late 1920s. Her subsequent career, as prop for a movie, and as prison ship, becomes material for Robert P. Watson's The Nazi Titanic: The Incredible Untold Story of a Doomed Ship in World War II.
Thus Book Review No. 4. Cap Arcona entered service just as the Depression began, and with the coming of the Nazi government, making a crossing on a German-registered ship became something that Just. Wasn't. Done. On the other hand, putting the ship into service with the Strength through Joy political education cum recreation enterprise kept the ship afloat and in service. With the coming of war ... now, that's an expensive hull with deteriorating machinery, and thus its use as a floating prop. Josef Göbbels had the idea of discrediting the British by class-shaming their institutions, and what better way to do that than make a movie about the sinking of Titanic with a particularly National Socialist spin on Jewish plutocracy and British snobbery? Conveniently, there's a large, idle ship that's not going anywhere (not that it's going to carry troops to land in Britain anyway) at hand. Later, as the war starts to go badly for Germany, that large, idle, ship is not going to help evacuate anybody from East Prussia or the Baltic States. On the other hand, it might be a place to hold concentration camp prisoners until they can be evacuated (the Germans devoting resources to concealing their deeds, even at the expense of protecting civilians or conducting a fighting withdrawal).
A large ship, however, is a military target, and some of the largest, and most unremarked, shipwrecks of the War era involved German liners removing refugees only to be found by Soviet submariners and British aircraft. Thus came the end of Cap Arcona, just before the end of the War, at anchor off Lubeck. British ground forces were close enough by then to rescue a few survivors, who their German captors had left to die.
(Cross-posted to Cold Spring Shops.)