Major delays are expected across inner city and suburban lines as rail workers at City Circle and Redfern strike from 10am to 4pm, while suburban trains will run on reduced timetables.
Rail, Tram and Bus Union secretary Alex Claassens said he received a fresh deed on Wednesday for changes to a fleet of Korean-built intercity trains the union believes are not yet safe to operate.
He received the deed 10 minutes before Employee Relations Minister Damien Tudehope held a Wednesday press conference calling on the union to cancel Thursday's strike action, despite conceding an agreement had not yet been signed by either party.
"That was never going to happen," Mr Claassens told reporters on Thursday.
It would take at least 48 hours for union delegates to review and vote on the deed, and timetable-planning for the network would take just as long, he said.
However, the deed is better than the one the union received last week.
"I think it is doable," Mr Claassens said.
It could be enough to head off a ban on operating foreign-built trains - which make up the bulk of the network - next Wednesday, the final day of the union's planned August industrial action.
Another meeting is scheduled for later on Thursday, with unions and transport management negotiating a new enterprise agreement after the old one expired in May last year.
The industrial action comes two days after a chaotic Tuesday on Sydney's rail network, when services on the T1, T7 and T9 lines were affected, and the T5 line was prevented from operating.
Treasurer Matt Kean says the trains the union wants changed are safe but the government has agreed to modify to end the dispute.
"These are some of the most modern, state of the art trains you'll find anywhere in the world," he told Sydney radio 2GB on Thursday.
The union says the NSW rail network is not designed to run driverless or guardless trains in the same way as the Sydney Metro, where platforms are separated from approaching and departing trains.
The new trains utilise CCTV to monitor platforms, and the union wants the screens moved out of the driver's field of view to prevent distraction.
Guards on the train should monitor platforms in person, as well as using the cameras, the union says.
Mr Kean said the trains have been deemed safe by the Office of the National Rail Safety Regulator, however the union disagrees.
"All the rail regulator does is ensure that rail operators follow the correct procedures," the RTBU said.
"It does not determine whether any component of the trains, including how they are operated, are safe."
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