Fare-free day flagged after Sydney train failure chaos

Premier Dominic Perrottet has apologised for a communications glitch that paralysed Sydney trains during the afternoon peak and flagged a fare-free day to compensate frustrated commuters for the ensuing mayhem.

The rail network's digital radio system that controls communication between trains and the rail operations centre failed at 2.45pm on Wednesday.

The breakdown stranded 250,000 commuters trying to get home at the end of a steamy Sydney day, causing a ripple effect across the city's transport system as buses quickly filled up.

Every Sydney train was parked for 90 minutes, awaiting instructions before getting the all-clear just after 4pm.

I'm really sorry for the inconvenience that many faced yesterday," the premier told reporters on Thursday.

"I've made it very clear to the secretary of the department that my expectation is ...  there is a fare-free day to make up, in some way, for what occurred."

Sydney Trains chief executive Matt Longland also apologised, saying the problem was a failed component in one of the IT systems that had now been replaced after engineers worked through the night.

"The system is stable," he told reporters.

"We're confident that we will have a normal timetable operating today."

Transport for NSW chief operations officer Howard Collins said the communications system was designed to automatically switch over from one channel to another if there was a problem.

"But for some reason ... it did not operate as it should," he told Sydney radio 2GB on Thursday.

Commuters were stuck for up to 90 minutes on trains and platforms as buses were unable to pick up all the slack.

Some people were forced to pay exorbitant fares for ride-share services, which were charging a premium as demand soared.

Engineers were investigating why the key component failed and how to prevent it happening again.

"We're also going to ask independently for another team to have a look at it so we've got a real forensic understanding of the real root cause," Mr Collins said.

Labor's transport spokeswoman Jo Haylen said it was a core responsibility of state government to provide a safe and reliable train system, including backup systems and backup buses to get stranded commuters home.

"The lights were out across Sydney trains but they are also out in the transport department," she said.

"No one knows who's in charge, and passengers are paying the price."

Uber has apologised to customers who spent hundreds of dollars on fares during the outage, promising riders charged more than the surge cap would be refunded within the next 48 hours.

"We had no warning that there would be such a complete outage on the NSW trains network," a representative told Nine News.

"As soon as we became aware of what was happening, our teams immediately lowered surge that aimed to still incentivise those driver-partners who were helping Sydneysiders get home, while making rides more affordable for those stranded."