Emergency services say they were called to the line in Peterlee on Wednesday afternoon after reports of a casualty on the tracks.
North East Ambulance Service say they dispatched a double crewed ambulance, a clinical care manager and one crew from their Hazardous Area Response Team.
A man was pronounced dead at the scene.
The British Transport Police say the casualty’s family have been informed.
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A spokesperson confirmed in a statement on Thursday: “Officers were called to the line in Peterlee at 2.52pm yesterday (15 September) following reports of casualty on the tracks.
"Paramedics also attended however sadly a man was pronounced dead at the scene.
"His family have been informed, and officers are currently working to establish the full circumstances behind his death.”
Northern rail services between Newcastle and Middlesbrough were disrupted for a number of hours in both directions while the emergency services dealt with the incident.
Among stations affected along the route were Sunderland, Seaham, Horden, Hartlepool and Seaton Carew.
The company said in the social media message to its Twitter followers on Wednesday: “Services between Newcastle and Middlesbrough are being disrupted in both directions due to the emergency services dealing with an incident
“All stations between Newcastle and Thornaby will not be served. Disruption expected until 18:00.”
The exact location of the incident has still to be confirmed and an inquest into the man’s death is expected to open shortly.