On level 6, "Metro Mayhem" you will reach a station with an arcade. Get on the train at this station. Stand at the door and wait for a darkened station blocked by a gate to pass by. Note: Stay on the train if you miss it. Jump off the train at the darkened station, then find a hole in the gate toward the left. Once in the station, look for two vending machines. Blow up the crack in the wall with explosives and enter. Move through the bathroom and the infested train car. You can locate the secret exit to the "Top Secret" level at the end of the broken train car's track.