Feral Ghouls (also called 'zombies' by some people) are common enemies in the Capital Wasteland, and are found in especially darker areas like abandoned metro stations. Wasteland myth such as "ghouls are scared of sunlight" is fake. Try checking out the Warrington Train Yard. A few shots with a Hunting Rifle will usually take them down. The Combat Shotgun seems to be one of the most effective weapons to take down one of these 'zombies' and is now one of the weapons that is mostly used by people that are going into ghoul territory. Be carefull when you are meeting one of the more 'advanced' ghouls, such as the Feral Ghoul Roamer or Reaver, or the most dangerous, a glowing one. If I were you, I would avoid that one as they can blast radiation which can undoubtedly affect you. If your travels need you to go through these areas containing ghouls, always bring a Combat Shotgun or a Hunting Rifle and some radaway or rad-x to keep some of the radiation where they dwell in at bay.