The obvious implication is that the murderer, disguised as a conductor, boarded the train...

Hercule Poirot: The obvious implication is that the murderer, disguised as a conductor, boarded the train at Belgrade, made his way by means of the convenient passkey to Ratchett's compartment, stabbed him to death, planted the dagger and the uniform, and then departed, since the train was now halted in a snowdrift. Who was he? I am inclined to agree with Mr Foscarelli, who believes that he was a rival member of the Mafia, exacting private vengeance for a vendetta whose precise nature the Yugoslav police will undoubtedly identify. Dr. Constantine: But... is that all? Hercule Poirot: No. No, no, no, no. No, it is not. I said, here is the simple answer. There is also a more... complex one. But remember my first solution when I... when you've heard my second.