Authorities insist a new rail system is safe after commuters were stranded on a busy driverless train when a carriage door was left open between stations.
"The staff who noticed the door was becoming ajar acted very quickly," Sydney Metro chief Peter Regan said on Thursday.
The train was travelling through the city's north from Chatswood to Crows Nest about 8am during Wednesday's peak.
It's about a three-minute trip between the two stops and Mr Regan said the driverless train came to a halt before reaching Crows Nest.
While he could not say how fast the train was travelling, Sydney Metro trains can reach speeds of up to 100 km/h.
"It is a serious incident," Mr Regan said.
"It hasn't happened before and we are determined to ensure that the systems and procedures are reviewed."
Investigations are continuing, but Mr Regan said he understood staff did not fully follow procedures to manually shut the door.
NSW Premier Chris Minns told commuters there was no need to overreact.
"No ones' lives were directly in jeopardy. It's obviously a glitch in the system," he said on Thursday.
"There's been human error and the job here is to make sure it's not repeated, not to try and rake an employee over the coals."
The premier travelled to Campsie Station in Sydney's west with Mr Regan to announce testing had begun on the Southwest Metro Line.
The first train rolled into Campsie, marking the start of more than 65,000 hours of testing before the new line to Bankstown opens in 2026.
"A commonsense approach to these brand new pieces of infrastructure is really important, not an overreaction," Mr Minns said.
The premier said he was not proposing to change any arrangements for Metro trains after Wednesday's incident.
"The systems that we've built, we pay an enormous amount so that they can operate effectively and safely," he said.
"We want to make sure this problem is fixed, but we also want to make sure that people can use the Metro this afternoon."
Rail unions are calling for the government to ensure the Metro system is safe.
"It's a miracle that no one was hurt, or worse, on the Metro," Rail, Tram Bus Union secretary Toby Warnes said on Wednesday.
"It's clear that the Metro system doesn't have a mature enough safety protocol system in place to deal with issues like this."
