Southern Rail said a damaged rail at Littlehampton could cause disruption to services until 5pm.
It is affecting services from Littlehampton to Brighton and Bognor Regis.
A rail replacement bus service is set to run between Littlehampton and Bognor Regis from 3pm, calling at Ford and Barnham.
A Southern spokesperson said: “There will be reduced service into and out of Littlehampton station this afternoon.
“We have been advised of a damaged railhead which is affecting Platform 3 and Platform 4.
“From 2.30pm Network engineers will take a line possession blocking the use of platform 2 for approximately 90 minutes whilst they attempt to weld the damaged railhead.
“During the block period services to and from Littlehampton will be limited as services will only go in and out of Platform 1. We advise you to check journey planners prior to travelling this afternoon as service alteration and amendments will occur.”