A picture showing a girl lying on a rail track where trains pass at up to 85mph has prompted a safety appeal by Network Rail.
The girl and a friend were caught on CCTV at a level crossing near Horsham, West Sussex.
Network Rail also released images of two boys walking down the track at Bewbush, West Sussex.
The track operator said that it had seen an increase in youth trespassing in Sussex.
Nicola Dooris, community safety manager for Network Rail, said: "On that section of track trains often travel at 85 mph and at that speed, unlike cars, trains can take hundreds of metres to stop, a fact many young people don't realise."
The children involved in both incidents, which happened in May, have been identified and spoken to by British Transport Police.