The first electric trains on Scotland’s flagship route will be introduced nearly a year late because of delays to a major upgrade.
Passengers who have suffered significant disruption during the project will now have to wait until the autumn to benefit from quieter, smoother carriages.
A series of problems has twice postponed electrification of the main Edinburgh-Glasgow line being completed.
Network Rail chief executive Mark Carne is expected to tell MSPs this week that full electric services are scheduled to start in December – but it cannot yet be guaranteed. They will be provided by new Japanese-designed Hitachi trains, which also have more seats.
But industry sources said the first electric services – using trains switched from other lines – may now not start operating until as late as October.
These should have begun last December, but were first postponed until next month because of problems with the £795 million Edinburgh Glasgow Improvement Programme (Egip).
However, a further delay was signalled last month when Network Rail revealed faulty electrical equipment would have to be replaced, postponing the overhead electric wires going live two weeks ago.
The Scottish Government has described it as “wholly unacceptable”.
One industry source said: “I’m hearing the project is at least 20 weeks behind.”
The initial electric trains will also provide about six of the dozens of services a day on the 46-mile route via Falkirk.
Journeys are due to be cut by ten minutes to 42 minutes by December 2018.
Carne, who will update a Holyrood committee on the project on Wednesday, told Scotland on Sunday: “Our aim is have the [full] electric service by the end of the year.”
However, he said electrification projects were “incredibly challenging”.
Egip has caused major disruption such as the closure of the Winchburgh tunnel for six weeks in 2015, while the Queen Street tunnel was shut for associated work for five months last year.
However, Carne said that after overnight work on the line there had not been a single engineering over-run. He said the faulty electrical component had not failed before and was not new.
He said: “I have no hesitation to say we have done the right thing to replace it.”
Train drivers’ union Aslef said the project had been badly handled. Scotland secretary Kevin Lindsay said: “It’s disappointing the introduction of electric services may be delayed.
“The Scottish Government and Network Rail should hang their heads in shame as this major infrastructure project has been mismanaged and lacked political leadership.”
Transport Scotland said: “Network Rail has provided an update, which will be shared with Parliament in the coming days. It would be inappropriate to comment further until this.”