When Visitors Attack!

 is a quest in the Fallout: New Vegas add-on Old World Blues.

Quick walkthrough
Ask Dr. Klien about previous visitors to the Big Empty. He will say that a visitor (presumed to be Father Elijah) attacked Dr.0 and Dr.8, destroying 8's voice chip, and escaping from the Big Empty, while also managing to take control of the trains in The Big Empty and crash them all, making an exit for themselves. The player then must ask Dr.0 about the attack. Quest concludes.