Hampden Park level crossing was stuck shut due to a fault preventing the barrier from working.
The barrier went down at 10.25 and was stuck for nearly an hour.
A Network Rail spokesperson said, “Public safety is our priority, so the barriers are being held in the down position until the fault is fixed.
“Engineers were working to fix the problem.”
During the closure, Sussex Police diverted traffic and pedestrians via alternative routes.
The Network Rail spokesperson added, “We would like to apologise for any inconvenience caused.”