This signal is first discovered when the Sole Survivor activates the relay dishes at relay tower 1DL-109. The source of the signal can then be tracked to a blue train car to the south of the relay tower.
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If anyone can hear this, my son and I are trapped and need help. We were being chased by... something, but were able to escape into this train car. It knows we're in here though. Thomas is hurt and needs...Oh no, it got the door open! Run Thomas, get out!
This has been a pre-recorded message. Message repeats in three seconds.
The Distress signal appears only in Fallout 4.