The Mystery of the Blue Train

Hercule Poirot boards Le Train Bleu heading for the French Riviera.  On board he meets Katherine Grey who will be enjoying her first winter out of England thanks to a recent inheritance.  Grey in turn meets Ruth Kettering, an American heiress leaving an unhappy marriage to be with her lover.  The next morning, Ruth is found dead in her compartment and her "Heart of Fire" ruby given to her by her father is missing.  Her father, the American millionaire Rufus Van Alden convinces Poirot to take on the case.

The police suspect that Ruth's lover, Comte de la Roche is guilty of her murder, but Poirot suspects otherwise.  The detective suspects Derek Kettering, Ruth's husband, who is on board but claims he hasn't seen his wife.  A cigarette case inscribed with the letter K found in the dead woman's compartment casts further doubt on Derek.  When Poirot discovers that the famous jewel thief Le Marquis is on board, he realizes that the murder and the jewel theft may not be linked.

The dancer Mireille, on the train with Derek, reveals that she saw him leave Ruth's compartment around the time of the murder.  Derek is arrested but Poirot remains unconvinced.  He asks Van Alden and Knighton to come on the train with him to recreate the murder.  Knighton is really Le Marquis, the jewel thief, but no one suspected him since he was supposed to be in Paris at the time of the murder.  It turns out that the K on the cigarette case stands for Knighton, not Kettering.

https://en.wikipedia.org/wiki/The_Mystery_of_the_Blue_Train