Franklin Metro Utility

 is a few Metro utility tunnels, part of the Metro track tunnels, and a cave system. It's slightly radiated, heavy in places, and partially waterfilled.

The tunnel loops back onto itself, and you can drop down a hole in the floor to reach the beginning after you've been through.

Inhabitants

 * A lot of Feral Ghouls and Glowing Ones.

Notable loot
(It's also rumoured to have a Tumblers Today, but it remains unconfirmed. Please update if you happen to find it.)
 * Guns and Bullets on a desk, downstairs towards the radiated/waterfilled area (A).
 * A very useful Advanced Radiation Suit and a Mini Nuke at the end of the waterfilled tunnel (B).
 * Burnmaster in a heavily radiated end of the tunnel, in a freight car (C).
 * U.S. Army: 30 Handy Flamethrower Recipes in a bookshelf (D).