This is an old sci-fi/action short film (about 5-10 years ago). This is the plot: in a subway station, a woman in long coat gets on a train. There are three other people in the wagon: one lying on the seat, another one listening to music on his walkman and a an old guy with a walking stick, rhythmically knocking with it on the floor. The train departs, there’s a tunnel and darkness, and when the view lights up again, one of the guys is dead — killed by a stab wound in his neck. Another tunnel, darkness, then light — second dead, once again with a small round bleeding hole in his neck. Third tunnel, and another dead character. Finally, the identity of the killer and the reason for the killing is revealed (don’t want to spoil it) in a surprising plot twist.