British Transport Police have confirmed the trains were now running on the slow lines but there would be a knock-on effect on services until around 2pm.
A person has died following an incident on the railway line near St Neots station.
British Transport Police said the death was not being treated as suspicious and that a file was being prepared for the coroner.
The incident has caused major disruption to services on the East Coast Main Line and a spokesman for train operator Great Northern and Thameslink said services may face problems into the afternoon.
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A British Transport Police spokesman said: "Officers were called to St Neots station shortly after 9.30am this morning following reports of a casualty on the tracks.
"Paramedics also attended. However, sadly, a person has been pronounced dead at the scene. Officers are now working to identify the person and inform their family."
The spokesman added: "The incident is not being treated as suspicious and a file will be prepared for the coroner."
The Great Northern spokesman said the incident happened at 9.32 just to the south of St Neots station and had affected trains travelling in both directions.
He said trains were now running on the slow lines but there would be a knock-on effect on services until around 2pm.