The Iron Horse - Season 1 Episode 23
Appointment with an Epitaph

Six people from a stagecoach board Calhouns train, all in fear for their lives after being followed for miles by a notorious gunman, who also boards the train. Calhoun puts him off, but he keeps following, and wont reveal which of the people he has been hired to kill, leading each of them to paranoid fear which may be a worse killer than his gun.