Distress signal (Relay tower 1DL-109)

Distress Signal is a quest in Fallout 4.

Quick walkthrough
Found when extending the Relay Tower 1DL-109. Woman says she is hiding in train car with "Thomas." Shortly into recording she describes her hiding spot as a train car.

Detailed walkthrough
Go SE from the radio beacon until you meet the train tracks and find a train with several feral ghouls. The fourth car along (blue car, turned on its side) has a hatch to enter the train, hidden behind a bush. If you're having difficulty finding it, it's the only bush touching the train on either side. Inside next to a ham radio are the skeletal remains of the woman in recording, along with a locked crate.