Picking Up the Trail

Picking up the Trail is a Fallout 3 quest. It is also a Xbox 360/PC achievement and a PlayStation 3 Trophy.

Summary
Picking up the Trail consists of finding and gaining entrance to Vault 87.

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 Citadel 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 you were escorting are no longer in your care, and the objective is completed. You can now 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 your map marker to find him. He will give you permission to use the Vault-Tec computer terminal in the Archives, located in the A Ring. While you are in the lab, make sure to grab the copy of Duck and Cover! from the bathroom.

Use the Vault-Tec computer in the Citadel Archives to locate a G.E.C.K.
Your Pip-Boy directs you up the stairs to a door in the southeast corner of the Laboratory. This door leads to the Citadel - A Ring. Once in the A Ring, head right to the Vault-Tec computer on the south wall. As you go through the information on the vaults, you find 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 the copy of Guns and Bullets, which is in the room called 'the Den.'

Ask Scribe Rothchild for information about Vault 87.
Follow your Pip-Boy marker to Scribe Rothchild. He may be in the courtyard. When you ask him about Vault 87, he says that he will tell you where it is. Follow him to the wall map in the Citadel Laboratory. There he tells you that you can't enter Vault 87 through the main entrance because of lethal radiation levels. You must look for a back entrance through Lamplight Caverns. You also receive map markers on your Pip-Boy for all of the vaults except Vault 112, which is accessed via Smith Casey's Garage.

Lamplight Caverns

 * See also: Little Lamplight, Rescue from Paradise

Once you've arrived at Lamplight Caverns, Mayor MacCready will stop you at the gates of Little Lamplight. You can convince the mayor to let you 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 you chose this option, you must 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. You must ask Joseph to restore power to the computer terminal before you can use it. The door and terminal (average difficulty) are located in the Great Chamber.

Note that if you arrive at Little Lamplight before you complete The Waters of Life, you can get inside the caverns but you will not be able to ask MacCready about how to get into Vault 87.

Bugs

 * It's possible that this quest will be considered "completed" and move you onto the next quest before you get into Vault 87. This can be a problem if this occurs before you ask MacCready to open the gate to Murder Pass and/or Joseph to restore power to the computer console, as the dialog 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 by "playing by the game's rules". In the PC version, however, one can use the console command to switch off collisions, which will allow you to simply walk 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 Wing 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 you to follow him then make no effort to move, even under duress. To fix this just reload and approach Rothchild at a different time of the day — preferably when an non-player character is talking to him and his back is turned — then begin the dialogue again. Another options are to push him until he is near the wall directly on the left of the door leading to the Mess Hall, or to drag Rothchild's body to the desired location by knocking him unconscious, or to disable then re-enable him using the console.
 * Sometimes Rothchild will disappear after you read the Vault-Tec computer. Your quest will tell you to talk to him but the quest arrow will point to the middle of nowhere.

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