Passengers at London Euston are facing major delays after a person was struck by a train near Milton Keynes.
Pictures posted online show hundreds of rush-hour commuters stranded at the major transport hub as lines came to a standstill.
Some lines began to reopen at around 5.40 pm, the station tweeted.
A Network Rail spokesman said: "Emergency services are dealing with an incident near to the railway at Milton Keynes Central.
"Major disruption is expected for the rest of the evening. Passengers are advised to check with National Rail Enquiries for up-to-date information."
The incident is believed to have happened at around 4pm, causing disruption to journeys between Euston, the Midlands and the North West.
National Rail Enquiries said passengers should allow for delays of up to an hour.
Replacement buses have been ordered to run between Watford Junction and Northampton and also between Wellingborough and Northampton.