Picking Up the Trail

 is a Fallout 3 quest. It is also an Xbox 360 and PC achievement.

Gain entrance to Vault 87.

Scribe Rothchild
Scribe Rothchild can usually be found in the lab of the Citadel. If you are in a hurry you can just access the Vault-Tec Terminal in the A Ring without consulting Scribe Rothchild. If you ask him, all he will tell you is the location of the Vault-Tec Terminal.

Once you've consulted the terminal, Scribe Rothchild will show you its location on the lab map panel. He also warns you about high level of radiations around the vault, which leads him to hint the location of Lamplight Caverns

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 check or by talking about butts with him if you have the Child At Heart perk. Otherwise the only way through Little Lamplight is to help rescue Sammy, Squirrel and Penny from Paradise Falls by doing the side-quest Rescue from Paradise, which is an Xbox 360 and PC achievement.

Once you get in Little Lamplight, there are two ways to enter Vault 87:
 * 1) Survive Murder Pass. You must speak with Mayor MacCready or Princess to get the gate to Murder Pass open.
 * 2) Learn that there is a door that won't open from Mayor MacCready. This is a back door to Vault 87. You can ask Joseph about the door terminal, but that is entirely optional. The door and terminal (average difficulty) are located in The Great Chamber.