Who is the killer in the book the murder on the orient express?

Hercule Poirot: a retired Belgian police officer & current private detective in England

Monsieur Bouc: former member of the Belgian police force & current director of the railway line

Dr. Constantine: the coroner aboard the Orient Express

Mary Debenham: Daisy Armstrong's governess & instrumental in planning Ratchett's death

Mrs. Hubbard: Linda Arden, famous actress & Daisy Armstrong's grandmother

Colonel Arbuthnot: a close friend of Colonel Armstrong, Daisy's father

Princess Dragomiroff: a Russian princess & Sophie Armstrong's godmother

Hector McQueen: Ratchett's personal secretary & friend of Sophie Armstrong

Mr. Ratchett: Mr. Cassetti, who kidnapped & murdered Daisy Armstrong for money

Countess Andrenyi: Sophie Armstrong's younger sister

Count Andrenyi: the Countess' husband & Sophie's brother-in-law

Cyrus Hardman: a private detective from New York; in love with Daisy's French nurse, who committed suicide

Antonio Foscanelli: the Armstrong family's chauffer

Greta Ohlsson: Swedish; Daisy's nurse; cries a lot

Hildegarde Schmidt: the Princess' maid & the Armstrong's cook

Edward Henry Masterman: Ratchett's valet; recruited by Hardman to help with the murder plot

Pierre Michel: the train's conductor & the French nurse's father