Damaged overhead wires have seen all trains in and out of Northampton either severely delayed or cancelled.
In the past hour, train operator London Midland has reported a section between Hemel Hempstead and Watford Junction is causing the blockage.
Some passengers at Northampton station are reporting either cancelled trains or delays of up to two hours or more in the southbound direction.
At 4.40pm London Midland tweeted: "Damage to overhead electric wires Hemel Hempstead - Watford Jct trains running through these stations may be delayed and/or cancelled."
Updates to follow.
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