Derailed is an achievement/trophy in Metro: Last Light which is awarded to the player for killing all enemies on the Revolution level. You can kill them any way, but they must be dead, not knocked out. There are more enemies is the dead end tunnel with railcars in it, and even more come as reinforcements when the first groups are dead. This should be unlocked after killing the last enemy who leaves the airlock. Below is a video guide if you are having trouble.
Note that contrary to the achievement's description, ALL Communists in Revolution must be killed, including the pair of unarmed workers in the first large room. Executing them will inflict Moral Point losses.