"Thought you'd doublecross us and get away with it, did you? Not on your life. We're out to GET you, Ratchett, and we WILL get you!"

When Poirot first interviews MacQueen in Part One, Chapter Six, he brings in two threatening letters that Ratchett had received as evidence that someone was hunting him down. This particular note highlights both the writer’s aggression and desire to enact revenge for some unknown misdeed. In this novel, the ideas of vengeance and justice go hand in hand, and while the threatening tone of this moment depicts revenge in a negative light, the outcome of the mystery challenges this perception. The later revelation that this note is a red herring further reinforces the idea that vengeful behavior may not always be what it seems.

"You have, M. le Comte, the makings of a very fine criminal," remarked Poirot dryly. "A great natural ingenuity, and an apparently remorseless determination to mislead justice." 

During his second interview with Count and Countess Andrenyi in Part Three, Chapter Four, Poirot gets them to admit to their real identities and explain why they lied during the first round of questioning. The Count argues that they altered the name on the Countess’s passport in order to distance her from the case, but Poirot only sees this behavior as an attempt to foil his investigation. At this point in the novel, Christie is still using the term “justice” in a more legal sense as Poirot aims to identify the criminal, or Ratchett’s killer.

"Certainly. I would do the same again. Her mother was my friend. I believe, Messieurs, in loyalty—to one's friends and one's family and one's caste.” "You do not believe in doing your utmost to further the ends of justice?" "In this case I consider that justice—strict justice—has been done."

Princess Dragomiroff approaches Poirot in Part Three, Chapter Five and identifies herself as the owner of the “H” handkerchief, admitting that she lied about her knowledge of the Countess’s identity in the process. She offers this reasoning for her lies, and this quotation provides one of the first explicit mentions of the distinction between legal and moral justice. Poirot views her lies as a hindrance to the process of identifying Ratchett’s murderer, but Princess Dragomiroff argues that killing Ratchett was the right thing to do.

"Why did you lie this morning?" "Business reasons. Besides, I do not trust the Yugo-Slav police. They hate Italians. They would not have given me justice."

When Poirot questions Antonio Foscarelli about his identity as the Armstrong family’s chauffeur in Part Three, Chapter Eight, Foscarelli emphasizes that he felt the need to protect himself from the police due to his Italian heritage. The question of what justice means comes into play here as he suggests that the Yugo-Slav police would treat him unfairly simply because of his ethnicity. His argument emphasizes the idea that even if the law is technically fair, those who enforce the laws are corruptible and not guaranteed to act in good faith.

"It must be not by chance, but design I remembered a remark of Colonel Arbuthnot's about trial by jury. A jury is composed of twelve people—there were twelve passengers—Ratchett was stabbed twelve times."

Poirot offers his solution to the mystery in Part Three, Chapter Nine, and he finally puts into words the collective dynamic at play among the twelve passengers on the train. The fact that this vigilante group takes on the symbolic identity of a jury, a legal entity with the responsibility of delivering a verdict based on the evidence presented to them, calls attention to the righteousness of their criminal act. While Ratchett escaped justice in America, the evidence of his brutal crime proves his guilt to the passengers, empowering them to deliver their own verdict.