Rail services between Eastbourne and Lewes are delayed this afternoon (April 3).
Southern Rail said a signalling fault means Brighton-bound trains from Eastbourne are having to run at reduced speed.
Passengers can expect delays until 3pm, the rail operator said.
A Southern spokesperson said: “We have been advised by Network Rail of a fault with the signalling system in the Polegate area towards Brighton. The fault in question has been confirmed as an ‘Axle Counter Failure’.
“This means the signalling system is unable to automatically verify if the next section of track past the affected signal is clear.
“The rail network is designed so that if a signal stops working, trains will stop before they reach it.
“Whilst this issue is ongoing, train crew will stop at the affected signal and contact the signalling centre to confirm their location, which at this point the signaller will verbally authorise the driver to continue towards the next signal.
“This process adds time to every trains journey, which means services can experience short delays.
“Network Rail specialist engineers are on route to site to investigate the issue.”