Right at the start of Depot you'll be on a railcar. It will make a couple of turns then straighten out and you'll see a light in the distance. Aim a rifle straight ahead at the middle of the tracks about chest-height and your crosshairs should turn red. Fire quickly, and if you kill the guard on the tracks before you get too close this achievement will unlock.
I've created a full Enlightened playthrough video showing almost every Moral Point in the game as well as pointing out items of interest, such as this achievement.
You can see the full Depot mission with all items of interest and collectibles at the 1:12:47
mark in the video.