At around 10.30am this morning, Southern tweeted: "It is with great sadness that we report a person has been hit by a train between Three Bridges and Horsham.
"All lines are blocked between these stations whilst the emergency services work to deal with this incident."
As one of the main routes connecting Sussex to London, the incident is having a knock-on impact on train services further afield.
According to the Southern Rail website, disruption to the network is expected to last until 1pm.
British Transport Police has been approached for more information.