You ride alongside the train at maximum speed, looking through each window as you go. You're starting to think you're going to loose the train when you see him; you couldn't miss the face of the man who killed you. You take your aim and take your shot. There's a loud bang, muffled by the steady chugging of the train, and your target is dead.
You slow down, letting your horse rest, and holster your weapon. He's dead. You got your vengeance. You won. You can't go back, but you have nowhere else to go, so you ride along the tracks into the sunset.