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 dancer Mirelle, who was on the train with her lover Derek but now spurned and vengeful, 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. This also throws suspicion on Derek when a cigarette case with the letter 'K' is found. Katherine says she saw Derek enter Ruth's compartment. He is suspicious of Ruth's 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 rubies, but Poirot does not think he is guilty. Ruth's maid, Ada Mason, says she saw a man in Ruth's compartment but could not see who he was. Ruth's father, the American millionaire Rufus Van Aldin, and his secretary, Major Knighton, convince 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 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.

2.4 The Blue Train, Nice and the Riviera.

Murder on the blue train poirot