All lines were blocked after a person was struck by a train between Alexandra Palace and Welwyn Garden City.
Disruption on Great Northern and Thameslink rail between London and Stevenage is expected until the end of the day, though the lines reopened at around 8pm.
Emergency services attended the scene, according to National Rail.
In an update, Thameslink said: "Trains between these stations may be cancelled, delayed by up to 60 minutes or diverted via Hertford North."
National Rail enquiries tweeted: "It is with great sadness that we advise that a person has been hit by a train."
According to its website certain Grand Central, Great Northern and London North Eastern Railway trains were affected as well.
Commuters also spoke of delays and crowded platforms at Finsbury Park station following the incident.
The Standard has contacted British Transport Police for further information.
National Rail has directed people towards The Samaritans and The Campaign Against Living Miserably (CALM) following the incident.
It said: "If you or someone close to you has been affected by this incident or other, there is always someone to talk to.
"You can contact the Samaritans on 116123 free at any time, on any phone.
"The CALM Helpline number is 0800 58 58 58, or 0808 802 58 58 in London."
