He asks Van Aldin and Knighton to come with him on the Blue Train to recreate the murder. He does more investigating and learns more information, talking to his friends and to Katherine, eventually coming to the truth.

Everyone is convinced the case is solved, but Poirot is not sure. Eventually, the avaricious Mirelle, who was on the train with Derek - with whom she had been having an affair but, now spurned, is seeking revenge against him - tells Poirot she saw Derek leave Ruth's compartment around the time the murder would have taken place. Poirot investigates and finds out that the murder and the jewel theft might not be connected, as the famous jewel thief 'The Marquis' is connected to the crime. Further suspicion is thrown on Derek when a cigarette case with the letter 'K' is found there. Katherine says that she saw Derek enter Ruth's compartment. He is suspicious of Ruth's estranged husband, Derek Kettering, who was on the same train but claims not to have seen Ruth. The police suspect that Ruth's lover, the Comte de la Roche, killed her and stole the ruby, but Poirot does not think that the Comte is guilty. Ruth's maid, Ada Mason, says that she saw a man in Ruth's compartment but could not see who he was. Ruth's father, American millionaire Rufus Van Aldin, and his secretary, Major Knighton, persuade Poirot to take on the case. The famous ruby, 'Heart of Fire', which had recently been given to Ruth by her father, is discovered to be missing. The next morning, though, Ruth is found dead in her compartment, a victim of strangulation. On board the train Grey meets Ruth Kettering, an American heiress leaving her unhappy marriage to meet her lover. So does Katherine Grey, who is having her first winter out of England, after recently receiving a relatively large inheritance. Poirot boards Le Train Bleu, bound for the French Riviera. The novel concerns the murder of an American heiress on Le Train Bleu, the titular 'Blue Train'. The book features her detective Hercule Poirot. The UK edition retailed at seven shillings and sixpence (7/6) and the US edition at $2.00.

The Mystery of the Blue Train is a work of detective fiction by British writer Agatha Christie, first published in the United Kingdom by William Collins & Sons on 29 March 1928 and in the United States by Dodd, Mead and Company later in the same year.

Poirot mystery