Test yourself! Take this The Mystery of the Blue Train Quiz | Single Story

Answer: Emma Harfield

The news of her sudden wealth prompted Lady Tamplin, who was her distant cousin, to invite her to spend some time at Nice with her. Katherine found this to be a suitable way of entering Society.

Answer: Rubies

Rufus Van Aldin had recently purchased some famous rubies for his daughter, among which was the historic 'Heart of Fire' worn by Catherine of Russia. He had warned her not to take them along with her to the Riviera lest she be murdered for them.

Answer: To find out complete information regarding Derek Kettering

Van Aldin had advised his daughter to file for divorce and wanted information regarding his soon to be ex son-in-law. Mr. Goby has also featured in other books like 'Elephants Can Remember' and 'Funerals Are Fatal' where he had been hired by Hercule Poirot to obtain information required for solving the case.

He has a peculiar habit of staring at various pieces of furniture instead of looking at the person he is talking to.

Answer: He pretended he was writing a book on historic jewels.

He had planned to substitute the original rubies with high quality imitation ones and hoped that even if Ruth discovered the theft, she wouldn't prosecute him.

Answer: Both of them

When Derek left Mirelle and told her that he intended to marry Katherine, she was filled with rage and went to the police to confess that she had seen him leaving Ruth's compartment. Katherine had seen Derek enter the compartment at night but was unsure if it had been Ruth's compartment or the one next to it.

Answer: False

Kitty Kidd, a former actress, was his accomplice. She had secured employment with Ruth Kettering under the alias of Ada Mason.

Answer: Derek Kettering

The French police suspected the Comte de la Roche from the beginning and were anxious to obtain evidence against him. However, Mirelle's testimony that she had seen Derek leave Ruth's compartment after she had been murdered led to his arrest.

Answer: True

On receiving Miss Viner's letter asking her to return to St. Mary Mead and live with her, Katherine decided to go back. Miss Viner had been diagnosed with cancer and the doctor had predicted that she would only live for a year at the most.
St. Mary Mead is also where Jane Marple, the other famous character created by Agatha Christie, lives, but she doesn't make an appearance in this book.

Answer: To hide the fact that she had been impersonated earlier on the journey.

Ada had worn Ruth's clothes and impersonated her to create an impression that Ruth had been killed a little after the train left Lyon. In reality, Ruth had been murdered when the train had stopped at Paris. Knighton had initially planned to make the Comte de la Roche the scapegoat. But after he met and fell in love with Katherine and realized she had feelings for Derek, he decided to throw suspicion on him.
Incidentally, the name of the conductor who discovered Ruth's body was Pierre Michel, the same as that of the conductor in another Christie novel 'Murder on the Orient Express'.

Answer: Richard Knighton

Hercule Poirot realized that Richard Knighton had been in America, England and France at the same time as the various robberies credited to the Marquis had taken place. Richard also spoke fluent English and French. The only thing that set them apart was that Richard walked with a limp due to a war injury. Poirot found out that the limp was faked and this clinched the case against Knighton.