There are a total of 5 people that you must save in this level. Thankfully they are only grouped into two. You will need to complete this as stealthy as possible, as raising the alarm will void the achievement. You can reload checkpoint if this happens. The first woman you must save is in a train car off to the of the main tunnel. You should hear her talking with two other men. Sneak up to the first, kill him (or knock him out) and do the same for the next guy. When they are both taken care of, she will sit or lie there.
Your next four targets are in the same room together. As you make your way through the tunnel, you'll come to a fork. Continue down this fork until you reach the entrance to the Bandits' main camp. Here, work your way through until you reach a door. When you open this door, a bandit will jump you. Quickly tap and he will die. Upon his death, the woman and child will run out the door and the achievement will be unlocked.