Emergency services are dealing with an incident between London Euston and Watford Junction, which is causing chaos for commuters and significant disruption between the stations.
British Transport Police said it was called to an area nearby London Euston station "following reports of a person being struck by a train".
A spokesperson said: "Our officers attended along with colleagues from the London Ambulance Service and a man was treated at the scene by paramedics for injuries to his leg and was taken to hospital. An investigation is ongoing into how the man came to be on the tracks."
London Midland, London Overground and Virgin Trains' services were all affected. London Midland has provided alternative routes for delayed passengers.
National Rail said services may be cancelled or delayed by up to 60 minutes. All lines were blocked, but have now re-opened though services are still delayed.
A spokesperson for Network Rail, which manages Britain's rail infrastructure and major stations, including London Euston, said:
Owing to emergency services dealing with an incident near Euston station earlier this evening, services into and out of London Euston station have been disrupted.
Passengers should check with their train operator before travelling as we anticipate disruption will continue for the remainder of this evening.
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