Emergency services attended the scene, close to Lostock Station near Bolton, at 6.20am, before the day's first service was due to depart.
Paramedics and British Transport Police located a body near the railway line, but the person was pronounced dead at the scene.
The cancellations affected dozens of services between Preston and Bolton, including Chorley and Buckshaw Parkway.
Blackpool commuters and holidaymakers travelling to Manchester Piccadilly and Manchester Airport were also affected after connecting services at Preston were cancelled.
A spokesman for British Transport Police said: “Officers were called to the lines close to Bolton station at 6.20am this morning following reports of a body close to the tracks.
“Paramedics also attended, but sadly a person has been pronounced dead at the scene.
“Officers are currently working to establish the circumstances behind their death.
“Anyone with information is asked to contact BTP by texting 61016 or calling 0800 40 50 40 quoting reference 79 of 5/2/19.”
The disruption to rail journeys continued for four hours until services resumed at 10.45am.
Speaking earlier this morning, a spokesman for Northern Rail said: "Due to the emergency services dealing with an incident near the railway between Preston and Bolton, all lines are blocked
"Cancellations and delays to these services are due to the emergency services dealing with an incident near the railway."
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