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 in charge and the train's police officer to gather all the passengers, including himself, in the train's main restaurant. But before the door to the restaurant is locked, you receive a message from Mr Poirot saying: “Don't come into the restaurant with the others. If I'm not mistaken, the murderer is here with us, but it's not proof of his guilt. I've left a letter for you, with all the information I know so far, and I'll stop everyone for an hour and pretend to interrogate them so you can find him! Good luck!”