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With exclusive access, this returning series follows the construction workers of Crossrail as they battle to finish the final stages of the new Elizabeth Line underground railway beneath the streets of London.

Costing over £15bn and stretching 120km across the capital, this extraordinary construction project is one of the biggest in Europe and one of the most ambitious engineering feats in Britain since the time of Brunel.

The final part of the series picks up immediately after the shock news that the Elizabeth Line has been delayed by over a year and will cost hundreds of millions of pounds more than planned. The show discovers how engineers, technicians and train staff must pull together - working on borrowed time - to build and fit out ten brand new stations, learn to operate the new trains and test out the new 21km twin tunnels under London, before it can open to the public. At stake are the reputation of the engineers, the reputation of the new railway and the reputation of British engineering.

Project manager Lih-Ling Highe is back to lead the installation of thousands of kilometres of vital power cables through the largest station on the Elizabeth Line – Tottenham Court Road. Tottenham Court Road is also expected to be one of the busiest underground stations attracting 200,000 passengers a day. If the cables are damaged or incorrectly wired, the station cannot be switched on and the project will be delayed once more.

At Bond Street Station, beneath London’s most expensive shopping street, Tim Weihen and his team are tasked with installing three 65m-long escalators – the longest on the railway – in the tightest of shafts so passengers can get to the trains. In a nail-biting process, the team must lower the bulky escalator sections, weighing up to seven tonnes, over the edge of a 30-degree drop in order to build all three escalators on time.

One of the world’s most famous train stations – Paddington Station – is getting a new Elizabeth Line station hub next door, and site manager Cynthia Myndhardt is in charge of building and fitting it out. The most daring feature of the station is a 130m-long great glass canopy to soar above the site and help plug it into Brunel’s original grade I listed station – a nerve-wracking feat considering any damage to the historic building could result in a prison sentence. 

The show also meets up with new recruit Rochelle as she continues her quest to become an Elizabeth Line driver. We follow her on her most stressful challenge to date – driving 1,500 passengers from Shenfield, Essex, to central London - in rush hour. Ahead of her lie 32km of busy track, 73 warning signals and 16 strict speed restrictions to follow, all of which must be intensely monitored along her route.

The episode ends with a second shock announcement that the project must delay again and that an even bigger bailout of £2bn is needed. With station construction, tunnel fit-out and train testing still incomplete, the fate of the now £17bn railway hangs in the balance.