Arlington utility is a part of the DCTAMetro system. This utility route is accessed through the southern collapsed rail tunnel of the Arlington Cemetery North station. This tunnel and cavern is a raider settlement and connects to a section of sewer via hole in the wall. This sewer has sandbag and cinder block defenses and is infested with Mirelurks. The large door at the southern end connects to DCTA Tunnel 014-B Potomac.
U.S. Army: 30 Handy Flamethrower Recipes is found on the second floor in the "kitchen area". Coming from DCTA tunnel 014-B Potomac, it's past the caves, when you enter the tunnels again full of raiders. The first stairs on the right, on a shelf. From the Arlington Cemetery entrance, after you pass the "White Line" sign take your first left (a four way junction), then a right (a T intersection). The kitchen area will be on your right.
When you enter from the DCTA tunnel 014-B Potomac entrance, there is a first aid box with a Rad-X and a stimpak in it to the left, when you pass behind the sandbags. Two more stimpaks are on the table to the right in the same area as the sandbags (this time behind some cement blocks).