Murder on the Orient Express

There is a murderer on this train... 1 passenger has been missing since yesterday! Mr. Hercule Poirot advises the conductor and the police officer to gather all passengers in the main restaurant. But before the door is locked, you receive a message from Poirot: "Don't come into the restaurant with the others. If I'm not mistaken, the murderer is here, but the proof of his guilt is not. I have left a letter for you with all the information I know. I'll stop everyone for an hour and pretend to interrogate them so you can find him! Good luck!"