Picking Up the Trail

Picking Up the Trail is the ninth main quest $$ It is also an Xbox 360/PC achievement and a PlayStation 3 Trophy.

Escort Doctor Li inside the Citadel.
Picking Up the Trail begins after Doctor Li speaks to Elder Lyons through the intercom, and the front gate to the Citadel is opened. The first objective is straightforward; escort Dr. Li inside the Citadel. There is no fighting, just follow her through the door to the Citadel courtyard, which is directly across from the front gate. Upon entering the courtyard, Elder Lyons approaches and Dr. Li speaks with him about Project Purity and what recently transpired, and requests his help. At this point, Dr. Li and the three men that were being escorted are no longer in the Lone Wanderer's care, and the objective is completed. It is now possible to ask Elder Lyons for permission to begin training to wear power armor, and he will consent.

Ask Scribe Rothchild for pre-War computer access.
The next objective is to ask Scribe Rothchild for pre-War computer access. He is located in the Citadel laboratory. Proceed across the courtyard to the laboratory door in the southwest corner of the courtyard. Then go speak to Rothchild, following the map marker to find him. He will give permission to use the Vault-Tec computer terminal in the archives, located in the A ring. While in the lab, make sure to grab a copy of Duck and Cover! from the bathroom.

Use the Vault-Tec computer in the Citadel Archives to locate a G.E.C.K.
Up the stairs to a door in the southeast corner of the laboratory, leads to the Citadel A Ring. Once in the A Ring, head right to the Vault-Tec computer on the south wall. When going through the information on the vaults, the Lone Wanderer finds out that Vault 87 was issued a G.E.C.K. under the non-standard equipment list. This objective is now finished. While here, make sure to grab a copy of Guns and Bullets, which is in the room called 'the Den.'

Ask Scribe Rothchild for information about Vault 87
Follow the map marker to Scribe Rothchild. He may be in the courtyard. When asked about Vault 87, he says that he will reveal where it is. Follow him to the wall map in the Citadel Laboratory. There he states that Vault 87 can't be entered through the main entrance because of lethal radiation levels. A back entrance through Lamplight Caverns must be found. Map markers for all of the other vaults except Vault 112 (accessed via Smith Casey's garage), will also be received.

Lamplight Caverns

 * See also: Little Lamplight, Rescue from Paradise

Once Lamplight Caverns is found and entered, Mayor MacCready will stop the Lone Wanderer at the gates of Little Lamplight. It is possible to convince the mayor to let the Wanderer in with a Speech challenge or with the Child at Heart perk. Failing that, the only way into Little Lamplight is to complete the Rescue from Paradise side quest.

Once in Little Lamplight, there are two ways to get to Vault 87:
 * 1) Passing through Super Mutant infested Murder Pass; if choosing this option, first speak with Mayor MacCready or Princess to get the gate to Murder Pass open.
 * 2) Asking Mayor MacCready about alternate paths will reveal a locked back door to Vault 87. Joseph must be asked to restore power to the computer terminal before it can be used. The door and terminal (average difficulty) are located in the Great Chamber.

Bugs

 * It's possible that this quest will be considered "completed" and begin the next quest before ever entering Vault 87. This can be a problem if this occurs before asking MacCready to open the gate to Murder Pass and/or Joseph to restore power to the computer console, as the dialogue options that prompt them to perform these tasks are no longer available after the quest is completed. Thus there is no way into Vault 87 at this point. In the PC version, however, one can use the console command to switch off collisions, which will allow simply walking through the gate blocking the way to Murder Pass. Once on the other side of the gate, use the  command again to re-engage collisions and then proceed on into Murder Pass.
 * Scribe Rothchild may disappear from the Citadel, preventing the player from being briefed and advancing the quest. This can be fixed by using the console command.
 * Talking to Rothchild when he is in the A Ring instead of the laboratory may result in a bugged and broken quest. Making him unconscious by shooting him can sometimes fix this.
 * In some instances Rothchild will tell the Lone Wanderer to follow him then make no effort to move, even under duress. To fix this, reload and approach Rothchild at a different time of the day — preferably when a non-player character is talking to him and his back is turned — then begin the dialogue again. Other options are to push him until he is near the wall directly on the left of the door leading to the mess hall or the wall map, to drag Rothchild's body to the desired location by knocking him unconscious, to disable then re-enable him using the console, or to repeatedly wait.
 * Sometimes Rothchild will disappear after the Lone Wanderer reads the Vault-Tec computer. The quest arrow will point to the middle of nowhere.
 * When MacCready grants access to Little Lamplight, the dogs and children may immediately be hostile, making the quest impossible to complete.