The industrial action threat was the latest chapter in the Rail, Tram and Bus Union's ongoing concerns about the New Intercity Fleet.

The union flagged concerns on Thursday about planned testing of the new trains today.

"We've been warning the government of serious safety concerns on the New Intercity Fleet and now we regrettably have no option but to take action to protect the community and our members from these trains that are a safety disaster waiting to happen," RTBU NSW Secretary Alex Claassens said.

On Friday the union said Transport for NSW had cancelled the planned readiness testing which had prompted their concerns would not proceed this week.

"Rail workers in calling this stoppage did so to ensure the railway network remained safe for commuters and after this unsafe testing was cancelled have agreed to go back to work and ensure Intercity services return to normal as soon as possible," the union said in a statement.

RTBU NSW Secretary Alex Claassens said it was disappointing "rail workers had to stand up as the last line of defence in protecting the community."

"I look forward to upcoming discussions where safety concerns will be front-and-centre and railworkers views will be heard," he said.

Earlier Transport for NSW had advised no passenger trains would run on the region's main line on Friday due to the industrial action.

A travel alert was issued about 3am on Friday advising all services on the state's intercity lines had been suspended.

It included the Central Coast and Newcastle line, as well as the South Coast, Blue Mountains and Southern Highlands.

"Due to industrial action, all NSW TrainLink Intercity services have been suspended for Friday 27th November," the travel alert said.

"Alternative transport is not possible with customers advised to either avoid travel or to make their own travel arrangements.

"Customers should monitor transport apps for up to date travel information."

No alert was issued for the Hunter line, but it was referenced in a NSW TrainLink post on Twitter.