The opening of the Elizabeth Line, the 15 billion-pound ($19.5 billion) London railway that will connect Heathrow Airport to Canary Wharf, has been delayed by at least nine months to allow for more safety testing.

The start date, previously set for December, has been moved to autumn 2019, Crossrail Ltd. said Friday. The builder said it needs more time to prepare the city’s central tunnels and to ensure reliability. Trains are currently being tested east and west of central London, as well as in tunnels around Heathrow airport.

“The Elizabeth line is one of the most complex and challenging infrastructure projects ever undertaken in the U.K.,” Crossrail Chief Executive Officer Simon Wright said in a statement. “We have made huge progress with the delivery of this incredible project but we need further time to complete the testing.”

As planned, the line will ultimately stretch from Reading west of London to Shenfield in Essex, and lift central London’s rail capacity by 10 percent to 200 million passengers annually. It is expected to boost the economy by about 42 billion pounds.

The train will allow passengers to travel across the capital faster than the existing network. Travel between Paddington station near central London and Canary Wharf, London’s second business district, will take only 17 minutes.

For more, see: London Heathrow Express Train to Downtown is Getting Competition.

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Photo Credit: Employees inspect a section of Crossrail track in a tunnel near Whitechapel station in London. Chris Ratcliffe / Bloomberg