A man was hit by a train at London Road station in Brighton.
Trains were reportedly delayed after the incident at about 7.20pm on Tuesday.
Trains between Brighton and Haywards Heath and Lewes were expected to be delayed by up to 60 minutes.
Southern passengers could use First Capital Connect services between Haywards Heath and Brighton and also on Brighton and Hove Buses between Brighton and Lewes, Eastbourne and Seaford.
Passengers were left trapped on trains as no services were moving.
East Sussex Fire and Rescue were at the scene, along with police officers. An air ambulance is understood to have landed in Preston Park.
One witness to the accident was shaken up by the incident.
Student Flo Murray said: "As we were walking through the tunnel under the station we saw a drunk man in a brown duffel coat.
"When we got to the platform he stepped off the platform and started walking up the tracks.
"My boyfriend ran after him along the platform and shouted at him but he seemed intent on what he was doing.
"The train came in breaking and honking its horn. The man hesitated for a moment before lying down and the train just rolled straight over him.
"We are really shaken up about it."
Southern Railways said: "Services between Brighton and Eastbourne are being disrupted due to a person under a train at London Road Brighton.
"Brighton & Hove buses are accepting Sothern tickets in the area."
On Twitter Brighton Station wrote: "PERSON HIT BY TRAIN LONDON ROAD. Get #12 bus for Newhaven, Seaford or Eastbourne, 25 for Lewes or via Haywards on train."
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