Southern Rail reported significant rail delays after a road traffic collision at the South Farm Level Crossing barriers shortly before 6.30pm.
"Network Rail staff and the emergency services have attended and have made it safe for trains to operate," a Southern spokesperson said.
"The emergency services are remaining on site, providing assistance to those involved."
All train lines were blocked due to debris on the railway, Southern said.
Its statement on social media read: "Our Control teams are working on alternative transport, so you may want to delay travel until we have this in place.
"Your intended train is likely to be cancelled or delayed at this time."
Train tickets are being accepted on buses whilst the rail delays remain.
Southern warned that trains which were en-route to Worthing were likely to terminate before they get to this part of the network.
A spokesperson added: "If you have not yet departed, we expect your journey to take an additional hour to complete."
At 7.10pm, Southern said it was in the process of moving trapped trains from the Worthing area.
Five minutes later, it was given the all clear to resume services 'at their regular line speed via Worthing'.
The spokesperson added: "Due to the number of trains/crew affected, your intended train may still be cancelled or delayed."
Lines have since reopened but trains between Brighton and Angmering may still be cancelled or delayed by up to 25 minutes. Disruption is expected until 8.30pm.
Trains are able to run once again between Brighton, Hove and Littlehampton but Southern said delays and cancellations 'may still occur', adding: "Southern are working hard to get services back to the booked timetable."