The £130m upgrade on the London Overground between Gospel Oak and Barking is facing more delays, after Network Rail said earlier this year work hadn't been finished on time.
It has postponed the announcements of when further work will be necessary, now saying a plan won't be released before 8 June.
While Transport for London (TfL) had originally planned for electrification of the line to finish by the end of next month, it was announced in February that the work had not been finished on time and more needed to be done.
Network Rail said at the time it was pulling together "a robust plan" to complete the work before new trains arrive early next year, which would mean further closures required over a series of weekends and likely involve a full closure for a period later in the year.
While Network Rail had intended to confirm future closures by the end of April, a spokesperson said it needs "a little more time to finalise our plan in discussion with stakeholders".
Delays to the work, which started in July last year, included incorrect design and late delivery of the huge masts and structures to carry power cables, with TfL saying it was "extremely disappointed" and would be seeking compensation from Network Rail.
Jonathan Fox, TfL's director of London Rail, said: "We continue to work with Network Rail to understand how they will complete the remaining work to electrify the Gospel Oak to Barking line with the absolute minimum further disruption to our customers. A number of options are being considered and we will make a more detailed announcement in due course when Network Rail can assure us that plans to complete the delayed works are robust.”
Network Rail's route managing director for Anglia, Richard Schofield, said "a full review into what went wrong" had already begun when the project delay was announced in February.
The line had already been closed for eight months to passenger services for the work to allow for the introduction of eight four-car electric trains.
"Between TfL and Network Rail something has gone seriously wrong with the electrification of the line. This has been increasingly clear for many months," said Caroline Pidgeon, Liberal Democrat member of the London Assembly and leader of its transport committee.
Passengers were promised a year ago that the upgrade work would be finally completed by the end of June 2017.
A year later Network Rail is now blatantly seeking to use the General Election as a figleaf to cover up their incompetence.
The route undergoing an upgrade: