Find the Hidden Ammo in the abandoned part of the station on the QUARANTINE level.
After heading through the guarded gate past the crowd of people, you will enter the "abandoned" portion of the city. There are a total of 4 caches that you must find. Each one contains 10 MGR. The locations are listed as follows:
After entering the area, begin to head down the short wooden walkway. However, walk far enough to where the door behind you will close. Now retreat back and collect the military grade rounds at the dead end.
After walking down the walkway, in the small open area, next to the gate with the yellow sign is an open window in the building to your left. The military rounds are in the open window of the building.
After proceeding past the previous cache, turn to your immediately left and see another opening inside the building. The rounds are in there.
The final cache is located inside another hole in a building past a small barrel filled with fire.