Trespassers on the rail line between Lewes and Eastbourne have caused disruption this morning.
All lines have now reopened, according to National Rail Enquiries, but disruption is expected until 10.30am.
Train services may still be cancelled or delayed by up to 15 minutes.
Those intending to travel are advised to allow extra time for their journeys.
A spokesman for National Rail Enquiries said: “Earlier today the driver of the 07.35am Brighton to Ore service reported a trespasser on the track.
“As a safety precaution trains came to a stand to protect all those involved.
“The trespasser is safely off the track and we are able to resume service.”