(CNN) — It's Europe's biggest infrastructure project, with 130 million working hours spent on it so far. As an engineering feat, it rivals the Channel Tunnel, which connects the UK to France. And it has the power to transform travel across one of Europe's busiest and most crowded cities.

Crossrail -- or the Elizabeth Line, to give it its official name -- promises to transform London with 100 kilometers of underground railway track cutting across the UK capital from west to east.

Walk through Central London, and evidence of Crossrail is everywhere: from the new station at Farringdon and the vast building sites at Tottenham Court Road and Bond Street, to the revamped Paddington Station.

London's first new underground line since 1999 (when the Jubilee Line was extended) comes with huge promise -- though the chronic delays to the project have caused political uproar. It is due to open at some point between October 2020 and March 2021 -- two years late.

When it finally does open, the Elizabeth Line will carry over 200 million passengers a year. Trains will pass through 42 kilometers of new tunnels stretching from Reading in the west to Abbey Wood and Shenfield in the east. In all, 41 stations will be served, 10 of which will be completely new.

A long time in the making

The Crossrail project is already two years late. John Phillips/Getty Images

The idea of a single line crossing London dates back to the 19th century. Post-war planning reports suggested such a line should be created, but it wasn't until 2004 that everything was put in place by government.

Work began in 2009, boring new tunnels through tight gaps between existing Tube lines and knocking down blocks in the city center to create new stations.

"The Elizabeth Line on its own adds about 10% to London's rail capacity," says Howard Smith, director of operations at Transport for London (TfL). "These are big trains, about 50% longer than any other Underground train. Twenty four trains an hour will run in each direction."

The Elizabeth Line will change the way London travels, Smith adds. Currently, London's main railway stations are on the edges of the city center. From there, passengers must decamp to buses or the London Underground.

"If you think of it, it's an odd idea to bring people into mainline rail stations on the edge of the center to then change trains and go down onto electric trains," he says.

The project has meant tunneling across London from west to east Courtesy Crossrail

"The really important thing is that [Crossrail] connects up places all on one line. You have one train that will take you from Heathrow directly through to the West End, on to the City, Canary Wharf and Stratford."

Once every section is open, visitors will be able to get an Elizabeth Line train from Heathrow Terminals 2/3 to Tottenham Court Road, in the heart of London, in just 28 minutes. Currently, it takes 38 minutes, using the costly Heathrow Express service, long billed as Britain's most expensive rail link.

The Elizabeth Line will also work with London's contactless Oyster Card system, as well as contactless payment cards and smartphones, meaning it will be cheaper and easier to get to London's hub airport than ever before.

And that's before considering the change for commuters, who will be able to get in and out of town easier.

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An engineering marvel delayed

Liverpool Street's new Crossrail station Courtesy Crossrail

"Crossrail is a massive project, in terms of the physical size of the Elizabeth Line across London, but also in terms of the amount of engineering squeezed into it," says Judith Ward, director of operations for the Institution of Railway Signal Engineers (IRSE), the international body for professionals who work in railway signaling and communications.

"In addition to the 42 kilometers of new tunnels they have built, they are delivering 70 new trains, working on three different signaling systems, 50 kilometers of communications cables and 41 new and updated stations, working with numerous contractors and consortiums."

It's this complexity that has led to delays. By now, the Elizabeth Line's trains were meant to be zipping Londoners across town. But 10 months on from its original launch date of December 2018, the Elizabeth Line is still a building project.

While the late 2020/early 2021 launch date does look likely, this is only for the central section from Paddington to Abbey Wood. Sections connecting Heathrow Airport and Reading to the west of the city and Shenfield to the east, won't open at the same time. Officials are only saying they will "commence as soon as possible."

But Howard Smith strikes an upbeat tone. "Bringing signaling systems, communications systems and the software that controls the signaling and the trains together, that's the bit that is taking longer to finish than was planned," he explains.

"It would be wrong to push on and do something that wasn't finished and open something that wasn't ready."

Smith adds that testing continues apace, with trains being run three or four days a week through the central section. "Each week we're testing a new bit of the system," he says. "There's an increasing number of trains down there. But we want to get it right before we throw the doors open."

Frustration for Londoners

The construction has seen major delays Crossrail Ltd

Needless to say, the delays to a project which was originally slated to cost £14.8 billion ($18.7 billion) and has since risen to £17.6 billion ($22.3 billion), have been met with frustration by politicians, regulators and everyday Londoners alike.

Having commissioned an independent report into Crossrail's failings, Mayor of London Sadiq Khan did not hold back when documents released in December 2018 showed how the project had been mismanaged dating back to 2013.

"I remain deeply angry and frustrated at the delays and cost overrun," said Khan at the time.

In August 2018, Crossrail bosses had told Khan that a launch date of December 9, 2018 remained in place. Three weeks later, they told him they needed to push it back a year.

In April 2019, they said they needed at least another 18 months.

The UK's National Audit Office has blamed an "unrealistic" schedule for the delays. "The compressed schedule, the contractual model, the loss of downward pressure on costs and the absence of a realistic plan were set against an atmosphere where 'can do' became unrealistic," it wrote in May 2019.

Ordinary Londoners have felt the delays keenly, too. A report for the London Assembly's Transport Committee cited the failure to open on time as leading to stress for new residents in Abbey Wood, an area of east London currently served by slow commuter services.

One anonymous responder said that she had bought a home in the area in 2017, in the anticipation that the new line would make her commute to the center of London easier, as well as giving her time to drop her child off at daycare once her period of maternity leave was over.

"I now have to try and navigate getting back to work with finding childcare that opens early enough and closes late enough for me to be able to drop my child off and still make it to work and back on time. This is highly stressful, not to mention expensive," she wrote.

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London is 'unrecognizable'

The construction has tunneled under central London Peter Macdiarmid/Getty Images Europe/Getty Images

Such a massive undertaking has inevitably had a visual impact on the city too. The upheaval had caused consternation among residents and local historians, even before work on Crossrail began in 2009.

Areas in central London have changed beyond recognition, especially around Tottenham Court Road and Soho, where the famous Astoria Theatre was knocked down to accommodate a new station.

"I'm a Londoner, and London always changes," says Howard Smith. "If we look at what London was 50 years ago, 100 years ago, it's unrecognizable.

"This was the biggest infrastructure project in the western hemisphere for much of its time, and for the size of the project it has been incredibly sensitive to how it has built things in the center of London.

"Quite a lot of cost and effort has gone into managing major buildings sites within yards of historic buildings. The effect it has had on the West End has been much less than people expected."

The overwhelming benefits of Crossrail mean Londoners and visitors to the city will doubtless forget the delays once the line opens. However, with a north-to-south Crossrail 2 project already being planned, such massive infrastructure projects will remain a key part of London life.

Hopefully, the failings of Crossrail will mean such a project won't take so long to go from planning to reality next time around. At least, that's what Judith Ward thinks.