Lithgow commuters will have to wait until 2024 before they travel on the new intercity train fleet.
Transport for NSW has confirmed the new trains will not enter service until next year - and even then Lithgow and the Blue Mountains will be the last area to have them rolled out, along with the south coast.
The updated timeline is the result of more modification works in preparation for the South Korean-built train fleet.
"Transport for NSW is working closely with train manufacturer Rail Connect and the Rail Tram and Bus Union (RTBU) to progress the modification work on the Mariyung fleet as quickly as possible and we are working with the RTBU and Office of the National Rail Safety Regulator to implement an operating model that will allow trains to enter service in 2024," said a TfNSW spokesperson.
"This includes changes to the trackside infrastructure and surveying has begun across the network to determine the scope of work required for infrastructure modifications."
The spokesperson said the new fleet has undergone testing between Central and Springwood and Central to Newcastle to examine mechanical and electrical systems, with preliminary testing of the crew doors now underway.
"These tests are needed to inform the detailed design of the modifications and the next steps towards getting these world-class trains ready for passenger service. As always, the safety of our people and passengers is our priority."
Blue Mountains MP Trish Doyle thanked the RTBU for "persevering with demands for necessary safety measures" for the new trains.
"People know that I've been banging on for years about the fact trains the previous government purchased didn't fit the tracks. I'm looking forward to the day that our commuting Blue Mountains community welcomes a modified new intercity fleet. I will work with the transport minister to ensure it arrives," she said.
The new fleet will be introduced on the Central Coast and Newcastle first. A timeline for the trains to operate on the Blue Mountains Line and South Coast Line will be determined when more trains are delivered and further testing is completed.