Young people in New York City are climbing on top of trains for thrills. The dangerous phenomenon, known as subway surfing, has been around for decades but is seeing a revival that sometimes ends in tragedy. There were six subway surfing deaths in New York in 2024. These tragedies are part of a global trend of deaths from social-media related challenges, as addictive app algorithms enthrall children and encourage users to make increasingly extreme content in a bid to rack up "likes". Our correspondents Jessica Le Masurier and Fanny Chauvin met the mother of a teenage boy who lost his life while subway surfing. She is one of a growing number of parents suing the social media firms Meta and TikTok for harming children.