First ideas

Thriller, Psychological/Crime, Film on train, Four people in scene, One is a cannibal, one is the victim of the attack from the cannibal and one is on train and notices the attack.

Person A on a train notices attack by a person on another person outside. A asks B if he saw that, B replies that he didn't and asks if A was taking drugs. Then we see outside the train where a there is a dead body on the ground, there is note next to it saying something.