Rail electrification between Cardiff and Swansea delayed
The electrification of the Great Western Railway line between Cardiff and Swansea will take longer than expected, a Network Rail report said.
The majority of the programme to modernise the line from London to Cardiff will go ahead as planned and should be delivered by March 2019.
But the route further west, due to be improved by 2018, will now not be done until between 2019 and 2024.
Great Western Railway said it was "disappointed" by the delay.
Last Friday an estimated £1.2bn rise in the project's cost was described by MPs as "staggering and unacceptable".
But on Wednesday, during the Chancellor George Osborne's spending review, the UK government reiterated that it was pressing ahead with the Great Western electrification project as part of key UK infrastructure investment.
The Network Rail report was published following the spending review and contained the conclusions of its chairman Sir Peter Hendy, who had been asked to look at the "deliverability and affordability" of the project.
He said: "Working closely with the Department for Transport (DFT) we have ensured that no infrastructure project has been cancelled and the bulk of the investment programme will be delivered by March 2019," Sir Peter said.
"Some projects will cost more and take longer than originally expected but we will see the job through to deliver better journeys for passengers.
"My review has clearly found that the original plan was unrealistic and undeliverable."
It added the Welsh government would be "making a decision on the scope and direction" of the Valley Lines electrification project as it is the primary funder.
Great Western Railway (GWR), which runs trains on the line, said it was too early to say what the full implications were for its plans to improve capacity and introduce faster, more frequent services.
"While the Super Express Trains will still be able to deliver some of the capacity improvements we planned for customers without full electrification, the full benefits will only be seen once Network Rail's electrification programme is completed," the company said in a statement.
"However, for now we will be working with the DFT to investigate alternative ways of delivering the full package of capacity and frequency improvements we promised in the new GWR franchise, despite the challenges."
Last week, a Public Accounts Committee (PAC) report said Network Rail's severe planning and budgetary failures caused delays and could double the budget.
Between 2014 and 2015 the estimated cost increased from £1.6bn to £2.8bn, not including the extension to Swansea.
Network Rail said changes had now been been made to control costs.
The line from London to Oxford and Bristol Parkway was originally due to be electrified in 2016, to Cardiff in 2017 and Swansea in 2018.