Commuters will be able to travel through London on the much-delayed Crossrail scheme before the end of June, the project team has said.
But only part of the full Elizabeth Line – as it will be known once in service – will initially open.
The new east-west line, which is set to increase rail capacity in the capital by 10 per cent, has ballooned in cost to almost £20bn and will finally open up to three-and-a-half years late.
Transport for London (TfL) has promised: “Passenger services will commence in the first half of 2022.”
Initially, though, the only service will be between Abbey Wood in southeast London and Paddington station on the western side of the city centre.
Andy Byford, commissioner of Transport for London, said: “The launch of this much needed and transformational addition to the transport network will be central to the recovery for London and the UK.
“Customers will experience a new way of travelling – with brand new, spacious step-free stations and new connections across the capital and beyond.
“There will be no better symbol of London’s renaissance from the pandemic.”
Mayor of London, Sadiq Khan, said: “The Elizabeth Line will transform travel across London and the South East, benefitting millions and supporting hundreds of thousands of new homes and jobs.
“Its brand new trains and step-free stations will help us deliver a modern, truly world-class transport system that allows us to support the growth in London’s population over the coming decades.
“That’s why I’m delighted it is on track to open in the first half of 2022.”
The line was originally scheduled to open in December 2018, with the budget set at £14.8bn. The cost has increased by almost one-third. Testing the system is currently costing around £2m per day.
When complete, the railway will include 26 miles of new tunnels and a total of 41 stations – 10 of them new.
When the key underground section through central London finally opens, travellers hoping to travel from Reading or Heathrow airport to Shenfield will need to change trains twice: at Paddington and at Liverpool Street.
The western half of the project will be connected through to Abbey Wood in “autumn 2022” with the final timetable – involving trains every five minutes in both directions – by May 2023.
Thousands of staff from TfL, Network Rail and other organisations will take part in training exercises involving police, fire and ambulance teams.
TfL predicts demand over the next decade will be 18 per cent lower than pre-pandemic forecasts.
The project team says: “The final cost of the project will not exceed the budget which TfL inherited when it assumed full control of the project, and London will ultimately fund the majority of the cost of the Elizabeth line.”
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