All lines between Littlehampton and Worthing were blocked following the incident in Lyminster Road, which left the barriers damaged.
Trains are now running but at a reduced speed.
A Southern Rail spokesman said: “Services are able to run but our teams are on site to monitor the trains whilst they run at reduced speeds through the affected area.”
Sussex Police confirmed officers were called to the scene at 12.55pm.
A spokesman said: “Officers attended and assisted Network Rail with a road closure.
“No serious injuries have been reported.”