Urgent repairs to the track between Horsham and Three Bridges means trains will be running at a reduced speed until 9pm.
Southern Railway said services from Horsham towards Three Bridges have to run at a reduced speed over the affected area, causing minor delays to services.
The line from Horsham towards Three Bridges will be closed temporarily for approximately 60 minutes between 7.30pm and 8.30pm. During this time, services towards London will not be able to run.
Southern said passengers should check their journey prior to travelling for service alterations and allow extra time to complete their journey.
On its website, Southern said: “A section of defective track has been identified at a level crossing between Littlehaven and Faygate. This is affecting services travelling from Horsham towards Three Bridges, which currently have to run at a reduced speed.
“Network Rail specialist engineers will need to close the line temporarily to give them time to make the repairs needed to the track.
“This is expected to take place between 7.30pm and 8.30pm.”