Note: There is an achievement/trophy that can be earned by making your way through the level without taking any damage.
Once the train arrives at the station, turn around and head to the back of the train for a Morality Point . Return to the control panel, exit the open doorway and follow your new partner across the platform to a locked door. Pull the lever on the wall to the right and go on inside. Again follow your partner as you climb the stairs to the top. When you reach the door at the top, look opposite and loot the dead soldier here for some ammo and +5 Military Grade Rounds . Enter the door.
Turn to the right and follow the other ranger until you find a ladder. She will hang out at its base. Instead of heading up right away, continue past the ladder and look down the small side tunnel to the left. Burn the cobwebs in here and at the end you will find a Diary Note [Note 1/1] . You will also earn a Morality Point here. Return to the ladder, follow the instructions to put your mask on and then climb to the surface.
Upon exiting, follow your partner until you reach the large stone archway. Afterwards you will be on your own. Once you are on the far side, head to the right and drop down to the area below.
Note: This next part is where the taking no damage portion of the achievement/trophy becomes a little tricker. There are a good number of watchmen that will attack at once and can overwhelm you pretty quickly. To do this easily, make sure to switch your ammo to military grade rounds, they will cause extra damage and will help you knock them down much quicker.
If this is too tricky however, you can reload the chapter and play on the easiest difficulty to reduce the enemy health.
At this point you will encounter your first Watchmen. There is a group of three along the Cliffside to the right. Fortunately your sniper friend will likely take one out for you so you can focus on the others. Once they are dead another 3 will appear on the very far side of the platform. Killing these will cause another 5-6 to spawn and approach from the left and far end of the platform.
Once they are all dead, make your way to the far end of the area and follow the passage here to the left. Follow the set path here all the way around until you see the target. It will run away, so crouch and follow it through the small tunnel it runs through. Upon exiting, turn right and follow it all the way until you reach a dead end.
When the target collapses, approach it for a scene. This will end the mission.
Note: If you made it through the level without taking damage, you will earn the achievement/trophy 'Not a Rabbit'.