HS2 was meant to be the railway of the future, an epic piece of engineering which would get Britain moving and help level up. But more than a decade on the costs have rocketed, key sections of the line have been scrapped, and the project is mired in uncertainty. Richard Bilton investigates what went wrong, talking to politicians, whistle-blowers and people living along the route to discover how billions of pounds of taxpayers' money was blown.