A casualty on the tracks at Arsenal Underground station was reported at around 6.30am today (January 15).
Police and medics, including the London Ambulance Service, were called to Drayton Park and a man was treated at the scene.
He was then taken to hospital, with injuries that are considered to be life changing.
The incident is not being treated as suspicious, the British Transport Police said.
There was no service for a while between Kings Cross and Arnos Grove due to emergency services blocking off the track.
Over three hours later, severe delays remain on the line.
Separately, there is also no service between Rayners Lane and Uxbridge due a shortage of trains as repairs are ongoing to fix damage caused by leaf fall.
A British Transport Police spokesperson said: “Officers were called to Arsenal underground station at around 6.30am this morning (January 15) following reports of a casualty on the tracks.
“Paramedics also attended, and a person was taken to hospital with injuries that are considered to be life-changing. The incident is not being treated as suspicious.”
A London Ambulance Service spokesperson said: “We were called at 6.24am this morning (Wednesday, January 15) to reports of an incident at Arsenal underground station in Drayton Park, N5.
“We sent a number of resources to the scene, including ambulance crews, members of our Hazardous Area Response Team (HART), an incident response officer and London’s Air Ambulance.
“Our crews treated a man at the scene, before taking him to a London major trauma centre by road.”
