Trespassers who continue to play near a reinstated train line in Nottinghamshire have been warned they are putting their lives at risk.
The numbers of youths seen close to the tracks at Tuxford has risen recently, British Transport Police said.
The High Marnham line was disused for many years but has been reopened by Network Rail.
The test track has trains running at speeds of up to 75mph (121km/h) at irregular intervals.
PC Kristene Mikkonen said: "As there is no scheduled timetable, trains will run as and when and youths should stay clear of this area.
"Those that continue to play line-side are risking serious injury."
Officers are stepping up patrols in the area and will take action against anyone who continues to ignore the warning, the police said.
The restored former colliery line includes 10 miles of track between High Marnham, near Newark, and Thoresby colliery junction, near Mansfield.