A secret report, prepared by UK consultants and obtained by Fairfax Media under Freedom of Information laws, said there were "simply too many underlying issues which have not been fixed".

It urged Transport for NSW to delay changes until early this year.

The report emerged as Sydney commuters on Thursday morning contended with disruptions on several city train routes due to "urgent overhead wiring repairs" at Flemington.

The new timetable was introduced in November 2017, but in January commuters faced two consecutive days of chaos when the system went into meltdown.

Around 100 services last month were cut to make the network more reliable after a high-level review.

The document was reportedly handed to Transport for NSW in March 2017.

It warned there was little room for systems to fail before the service might "disintegrate substantially".

Sydney Trains executive director Tony Eid told Fairfax Media the document was outdated and written about a draft of the timetable. There were nine further versions, he claimed.

Rail, Tram and Bus Union NSW Secretary Alex Claassens said anyone with insight into the network knew the timetable wasn't going to work.

"The government didn't consult with the workers - the people who know about how the systems work," he said in a statement.

"Had that happened, maybe we wouldn't have ended up in this mess."

Australian Associated Press