The number of US pedestrians who have been killed or badly injured while wearing headphones has tripled in six years, according to a study published Tuesday. More than half of the incidents involved pedestrians who were run down by trains. Two-thirds of the victims were males and under the age of 30. The research warns of “inattentional blindness” when wearing headphones, meaning a distraction that lowers the resources the brain devotes to external stimuli. The trawl of US accident statistics covered iPods, MP3 players and other musical devices. It did not include mobile phones.