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.