This novel is one big lie! Poirot eventually discovers that all of the characters in it are putting him on and acting out their parts to cover up the murder. But wait. Aren't all novels made up? Such is the nature of fiction. Murder on the Orient Express very interestingly aligns fiction writing with lies and deceit. Notice the many references to playacting and literature. It asks us to think about the differences between the two – and the similarities.
In this book, lying, like murder, is justified if it's for a good cause.
Sometimes truth is stranger than fiction.