The train car can be found via a hatch entrance at a ruined train yard to the south of Oberland Station, and directly southeast of Relay Tower 1DL-109. It can be located by following the distress signal that can be heard after the Sole Survivor extends the radio dish at the relay tower. The message is a request for aid by a woman and her son, whom are running from something chasing them, and hid inside an old train car compartment.
Keep heading southeast until the signal gets stronger (there will be less static the closer you get), and a hatch can be found on the northern side of one of the blue train cars that has been derailed and tripped over.
The interior consists of a single very room with the skeleton of a woman next to a Ham radio with a large amount of blood around her. The Sole Survivor may choose to turn off the Ham radio in order to end the broadcast.
It is unclear how long the woman has been dead for, what it was that was chasing her, or if her son "Thomas" ever made it out alive.
The train car only appears in Fallout 4.