County Sewer Mainline

 is a sewage tunnel which can be accessed from either inside the Sewer Waystation or a sewage grate along the Potomac, just north of a raider camp under the west point of the Arlington Memorial Bridge. It goes under Grayditch and Arlington Cemetery, but has no access point there.

It is the home of Gallo, as well a number of Feral Ghouls and Radroaches, and contains a large number of Workbench items.

Gallo
Nearby the Waystation access point (the right path after entering via the Waystation) can be found a hostile Ghoul named Gallo, whose room is located past several traps in the northwest region of the sewers. On Gallo's person can be found a Tales of a Junktown Jerky Vendor, and Gallo's Storage Key which is used to access a storage closet nearby. Gallo's room also contains a small Nuka-Cola collection, a safe containing a Nuka-Cola Quantum, as well as a pen of radroaches.

Gallo's Storage Closet
Gallo's storage closet can be accessed using Gallo's Storage Key, or by picking its lock.


 * 12x 10mm Rounds
 * 6x Abraxo Cleaners
 * 4x Baseballs
 * 1x Baseball Bat
 * 1x Baseball Glove
 * 1x Basketball
 * 2x Crutches
 * 1x Dirty Water
 * 2x Firehose Nozzles
 * 3x Fission Batteries
 * 2x Harmonicas
 * 3x Lawnmower Blades
 * 5x Leaf Blowers
 * 2x Lunchboxes
 * 2x Medical Braces
 * 1x Mentats
 * 2x Motorcycle Gas Tanks
 * 3x Motorcycle Handbrakes
 * 1x Nuka Cola
 * 3x Paint Guns
 * 2x Pre-War Book (With Gallo's Nuka-cola collection, not in the closet)
 * 2x Pressure Cookers
 * 1x Rake
 * 1x Rad-X
 * 40x Railway Spikes
 * 1x Stealth Boy (hidden in a box)
 * 24x Shotgun Shells
 * 2x Stimpacks
 * 5x Toy Cars
 * 2x Vacuum Cleaners

Sewer Management Main Room
The Sewer Management main room is located in the central region of the mainline. It is occupied by a number of radroaches and feral ghouls. It is located behind a locked gate which can be unlocked from a terminal nearby. Next to the terminal are two pre-war books and a Big Book of Science.

Sewer Management Storage
The sewer management storage closet can be accessed from a terminal behind a locked gate nearby.


 * 20x .32 Caliber Rounds
 * 1x Blood Pack
 * 1x Bobby Pin
 * 5x Conductors
 * 1x Harmonica
 * 2x Jet
 * 2x Med-X
 * 1x Paint Gun
 * 1x Pilot Light
 * 2x Radaways
 * 2x Sensor Modules
 * 12x Shotgun Shells
 * 7x Stimpacks

Sewage Tunnels
The sewage tunnels that compose most of the mainline are littered with the remains of dead mercenaries and wastelanders. These are often marked by the presence of feral ghouls. The careful observer will also notice boxes of ammunition nearby the remains.

Also in the tunnel between the sewer management main room and Gallo, there's a strange skull with glowing eyes stuck on a metal rod, with two smaller rods crossed just under it. It is probably just a random curiosity put there by the developers, but in any case it's worth checking out just for being a bit creepy. It could also be a reference to the Khan Raider clan from the original Fallouts.

Note: it is much easier to spot it with the Pip-Boy light turned off.

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Bugs

 * Just before unlocking the gate which houses a generator and a table I saved my progress but on reloading the game crashed, I deleted some old saves to try and fix it but it still froze when loading, however when I loaded a previous save outside the sewers the game ran smoothly. Meaning that this problem only occurs when inside the sewers system area, so be warned! --- qoucher: I second this, it was at a different area inside of sewer mainline, but the game had froze the system (360) and I couldn't reload my last save, luckily the autosave worked.
 * In the area down the tunnel from Gallo's room where there is a four-way intersection, there is a low concrete cinderblock wall. Items that you drop in front of it (towards the Arlington Memorial Bridge Exit) might disappear into the ground. So be careful
 * In a small room with a staircase near the exit to the Capital Wasteland, the player might find themselves SWIMMING down the staircase rather than walking. One bug that this player encountered had the game thinking the room and some immediate surrounding areas were filled with water, and he was able to swim all about the room and had an oxygen meter appear. Confirmed only on the Xbox360 version post-Broken Steel.