This quest begins as you go into the military bunker on Ravenholm. You will have to fetch a key from the General's office, sabotage the generator across the camp, and then escape out of the hatch back in the General's office. After this, you will return to Ollie, using the bell to go in through the back of the train station, and go up the stairs and lifts until you reach the top. When you are done speaking with Ollie, the quest will change, Band of Brothers will complete, and the achievement will pop.