Sydney trains have been brought to a standstill by a network-wide communication issue, with no indication when the problem will be fixed.
All trains were stopped at platforms on Wednesday afternoon due to the issue.
Transport for NSW advised passengers to consider alternative transport as peak hour approached.
"Trains on the Sydney rail network and intercity lines are stopped at platforms," Transport for NSW said in a statement shortly after 3.30pm.
"Crews are working to fix the issue as quickly as possible but there is no forecast.
"Passengers already travelling on the rail network are advised to listen to announcements and check information displays for service updates."
A Transport for NSW spokesman said the issue stemmed from a failure in the Digital Train Radio System, which occurred around 2.45pm.
"This is the digital radio system that connects the train crew with the signaller and our rail operations centre," he said.
"As a result for safety reasons we have had to halt trains at platforms until we can reboot the system."
All trains were able to be moved safely to platforms and no people were stranded in carriages, he said.
"Engineers are on site and looking at the system fault and determining if we implement a back-up system located at Homebush.
"We are advising people to avoid train travel now and we will update when the system is back up and running."
