The 15 Billion Pound Railway - Season 4

With exclusive access to Crossrail Ltd and Transport for London, this fourth and final chapter of the long-running documentary series follows the race to fix the railway's high-tech software and systems to launch the new Elizabeth Line in time for the Queen's platinum jubilee.

The 15 billion pound railway - now costing 19 billion and rising – stretches 120km across London in what has become an extraordinary construction project - one of the biggest in Europe and one of the most ambitious engineering feats in Britain since the time of Brunel.

For over ten years, cameras have followed the engineers, technicians and transport staff who are under pressure to deliver their section of the project, including construction fixes in the tunnels, safety checks of the stations and the testing the complex, bug-ridden train software, all in a bid to make it safe for passengers.

We follow 83-year-old site manager Jim Forrest – one of the oldest engineers on the project. He joined the Whitechapel Station build in 2009 and has seen it from tunnel boring and concrete pouring to constructing the station steel works and vital fit-out works. In order to open the station, Jim and the team must ensure it's safe in the event of a fire – learning the lessons of the 1987 King's Cross fire. They must conduct a crucial smoke trial on the platform to test the ventilation systems can extract huge amount of smoke should a fire break out. An overlooked detail in this phase could become a matter of life and death when the railway opens. 

Before any station on the line can open to the public, the railway must undergo a series of critical safety trials to judge whether Elizabeth line staff are ready to respond in the event of an emergency – from passengers falling sick on a train to the handling of a reported fire. 

We follow Lewis George and Rebecca Edwards, who are in charge of the trials and must coordinate the railway's largest evacuation trial involving 900 volunteers as they're rescued from a train in the tunnels and evacuated from Canary Wharf station – one of the largest central stations. If staff can't evacuate passengers safely, the teams could face more delays to the opening of the railway.

The end of the film sees the Queen visit Paddington Station and goes behind the scenes as the grand opening day of the Elizabeth line finally arrives, attended by Mayor of London Sadiq Khan, TFL commissioner Andy Byford, Crossrail CEO Mark Wild and more than 130,000 eager passengers and train enthusiasts jostling to board the first train services to depart. As the first train leaves Paddington, we check in with the men and women who helped build the line and featured across past series – station by station.