According to the backinfo I got, this happened in Japan and it was streamed live on Twitter.
It appears to have been filmed by a girl, who is wearing what looks like a school uniform. She left the phone streaming presumably on a bench as she waited by it for a train. When it approached the station, she jumped on the tracks in front of it. The phone did not capture the impact, but she most likely got fatally hit or run over.
Props to Best Gore member @nomar for the video: