On Feb. 23, 1950, a railroad signal worker discovered the badly mangled body of a man in a tunnel south of Salzburg, Austria. Among its torn clothes police found the diplomatic passport and service identification of U.S. Navy captain Eugene S. Karpe, who’d been returning to the United States after serving for three years as naval attaché in Rumania. It appeared that Karpe had fallen from the door of the Arlberg-Orient Express as the train sped around a curve in the dark of night. The train car had very small windows, and the doors had been locked automatically before the...