This also means the expansion to 10-car trains during peak hour promised in February last year will be delayed.

READ MORE: Rail commuters may struggle to find a seat

For several years the government had promised the new trains would reach the South Coast line by 2021.

However these "service improvements" have been quietly pushed back to 2022 as rail upgrades to the South Coast line begin.

This detail was mentioned at the bottom of a media statement from Transport Minister Andrew Constance about the $600 million rail upgrade contracts.

"Construction for the rail upgrades will begin in the middle of this year and be completed in advance of the service improvements, which are planned for late 2022," the statement read.

It's confirmation that the delay in the arrival of the first carriages of the New Intercity Fleet would have a flow-on effect for the South Coast line.

The rail upgrade announcement would see $300 million spent on improving infrastructure on the South Coast line to allow for 10-car trains and an extra off-peak service every half-hour.

"The work will support the introduction of new suburban and intercity trains and allow us to deliver more frequent train services, with less wait times and a more comfortable journey for customers," Mr Constance said.