Melbourne's train network was sent into disarray during the post-work rush on Thursday afternoon thanks to an unspecified "computer fault" that halted all trains across the network.
Metro Trains advised commuters just after 4pm that it was experiencing an "infrastructure fault" that had caused it to halt trains while it investigated the problem.
Less than half an hour later it changed its advice to blame a "computer fault" that had caused delays to all lines.
"No metropolitan trains are currently operating. Please listen for announcements," it said.
It told commuters not to use the train network and to seek alternative transport.
The disruption caused some passengers to be stuck onboard trains that had stopped in between stations.
"Please do not attempt to force doors open and alight from the train," Metro Trains said.
It said technicians were working to rectify the problem and restore services.
The company's website also appeared to experience issues.
A Metro Trains spokesperson was unable to provide detail on the technical cause of the problem.
On Friday morning Metro Trains chief Mike Haughton said a failure in the core train control system had meant operators could not "see" the trains, so it was shut down for safety reasons.
Metro Trains operates the suburban railway network in Melbourne. It services more than 415,000 passengers every week day.
Update 5:15pm: Metro Trains advised services were resuming, but warned of "major delays" as it restores the timetable.