Rail services between Northampton, Milton Keynes and London are severely disrupted after a person was hit by a train on Tuesday (October 19).
National Rail reported the incident between Watford Junction and London Euston at just after 8am.
All lines to and from the capital are likely to remain blocked for a couple of hours while emergency services deal with the incident, with delays and cancellations affecting southbound and northbound London Northwestern Railway and Avanti West Coast rush-hour trains.
Thousands of passengers are stuck on trains stranded along the route.
A spokesman for London Northwestern said: "We are still working to assess how this will impact our services but we would advise passengers to check National Rail Enquiries for the latest news.
"Please avoid travelling on this route until later if possible. If you must travel, you will be required to use alternative transport and your journey will take longer than usual."