The secret of the "blue train"

In the luxurious express “Blue Train”, next from London to the French Riviera, there was a tragedy. The daughter of the famous American millionaire Ruth Kettering was strangled in her own compartment, and all her jewelry, including an amazing ruby, disappeared. In the same train, the great detective of Erkul Poirot rode on the sea, who, of course, could not stay away from the investigation. Everyone around him only says that about the mysterious kidnapper of jewelry nicknamed the Marquis, and they are completely sure that this crime is his hand. However, Poirot received information that shortly before the unfortunate woman was discovered dead, her husband Ruth, Derek came out of her compartment