London's Crossrail is expected to be fully operational in mid-2022, according to the project's developers.
Europe's largest transport scheme had been due to open in December 2018 but the central sections will not open until summer 2021 at the earliest.
The line, between Berkshire and Essex, via central London, is set to cost £18.25bn - more than £2bn over budget.
Mark Wild, head of Crossrail Ltd, said: "We are doing everything we can to get this railway finished and open."
In November Crossrail Ltd, the company set up to complete the project, estimated up to £650m more was needed to finish the line.
Earlier this week, the outgoing commissioner of Transport for London said the line was expected to open in autumn 2021.
Crossrail Ltd confirmed it planned to open the central section between Paddington and Abbey Wood in summer 2021.
When open, the line will be known as the Elizabeth Line.
Crossrail Ltd said it was "increasingly confident" Bond Street station would be open before the central section was running.
TfL has lost between £500m and £750m in passenger revenue due to the delays.
Software development on signalling systems, which has been unexpectedly slow and plagued the project with delays, is making "good progress", according to Crossrail Ltd.
"Intensive operational testing" of the signalling and safety systems on the central section is due to begin in the autumn.
Mr Wild said: "I know that Londoners are deeply frustrated by the delays to the Elizabeth Line and we are doing everything we can to get this railway finished and open.
"But there are no shortcuts to delivery of this hugely complex railway."
A spokesperson for the mayor of London said: "TfL and the Department for Transport, as joint sponsors, will continue to hold the Crossrail leadership to account to ensure it is doing everything it can to open Crossrail safely and as soon as possible, and to mitigate further cost pressures."
Sean McKee, from the London Chamber of Commerce and Industry, said: "Of course safety is the first consideration, but it has to be appreciated that the delays to the project have been a blow to those businesses that made investment decisions based upon the previous timelines.
"These firms will now hope for the earliest possible opening date."