Nobody Left the Carriage

You all boarded the last night express, and somewhere between two stations the passenger in compartment four stopped breathing — a thin cut, a spilled cup, a window that was never opened. The train doesn't stop for ninety minutes, the doors are locked from the inside, and no one crossed the corridor without being seen. Yet everyone claims they slept. The conductor's log has a name scratched out, one ticket is a forgery, and the dead man mailed a letter naming his own murderer before he boarded. The next platform is coming fast, and it crawls with police. Cross-examine each seat, break the alibi that's too clean, and say who did it before the doors open and someone slips into the crowd.