Police say a 7-year-old boy who was selling candy on a Philadelphia train with his brother fell between two moving subway cars and died. Authorities say the boy was with his 11-year-old brother and a 26-year-old man on the Southeastern Pennsylvania Transportation Authority's Broad Street Line subway in northern Philadelphia on Sunday, the AP reports. Police say the boy was walking between cars when he slipped and fell onto the tracks. SEPTA Police Chief Thomas Nestel called the death "a horrible, horrible thing." He said the walkway is not safe while the train is moving. Transit officials say the National Transportation Safety Board is now investigating. Names of those involved haven't been released. (More Philadelphia stories.)