The line between Peterborough and London is currently shut after the incident, which occurred between Hitchin and Welwyn Garden City.
The closure is affecting Great Northern and Thameslink services, as well as long-distance operators including LNER, Grand Central, Hull Trains, and Lumo.
A Network Rail spokesperson said: “A person has been hit by a train between Hitchin and Welwyn Garden City.
“Whilst the emergency services carry out their work, all lines between Peterborough/Royston and London are closed.”
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Passengers are being advised to expect cancellations, delays, or last-minute alterations to their journeys, with disruption expected until 2.30pm.
Those travelling across the affected routes are being urged to check live departure boards and allow extra time.
They added: “If you are travelling through this area, it is likely you will need to use an alternative route to complete your journey.
“This will take at least 60 minutes longer than usual so please allow extra time.”
Tickets are being accepted on multiple alternative routes at no extra cost.
Passengers can use their tickets on other Thameslink and Great Northern services, CrossCountry services between Cambridge and Peterborough via Ely, Greater Anglia services between Ely, Cambridge and London, and London Underground services between all London terminals.
LNER tickets are being accepted on Avanti West Coast services between London Euston, Manchester and Glasgow Central/Edinburgh, as well as on CrossCountry, East Midlands Railway, Grand Central, Hull Trains, Lumo, Northern, and TransPennine Express on specified routes.
Passengers may be entitled to compensation if they are delayed.
Anyone struggling or in need of someone to talk to can contact Samaritans at any time, day or night, by calling 116 123 for free or visiting the Samaritans website.