Aboard a vintage luxury train on a voyage through Eastern Europe, the gourmet detective encounters a thoroughly modern murder.   The Danube Express was once the most famous train on the continent. Linking east and west, it was the fastest route from the Alps to the Black Sea, until airplanes and automobiles made it obsolete. When a group of savvy investors revived it in the 1970s, it became an Express only in name. A five-star hotel on wheels, it is now a luxurious icon--and it's celebrating its twenty-fifth anniversary.   Some of the most glamorous figures in the world have booked passage on this historic trip, and riding among them is London's gourmet detective, who has come to sample the Danube's famous cooking. But when a Hungarian actress disappears from the train, it turns out to be more than a publicity stunt. Soon it is clear that a killer lurks on the Danube Express, and plans on taking it all the way to the end of the line.