Operator East Midlands Railways said at 7am that all lines were closed south of Kettering after a trespasser was reported on the tracks.
Both InterCity services from Kettering and Connect 360 trains from Corby and Wellingborough are affected.
A spokesman said: "At present a Network Rail team are on site with the police and trying to locate the trespasser.
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"All trains running through the area in both directions will be delayed, the lines are closed south of the station, with no trains able to run at present, until the trespasser is removed."