Case in point: the Crossrail tunnel through Tottenham Court Road has about 85cm clearance both above and below it, and had to be constructed with the Undergroun

Case in point: the Crossrail tunnel through Tottenham Court Road has about 85cm clearance both above and below it, and had to be constructed with the Underground tunnels around it still in active service. Building new lines through relatively soft clay is really hard when you have hundreds of notable historical buildings that you cannot afford to have any subsidence (like, millimetres is enough to cause really damage), and then when you don't really even though where half the other underground pipes are exactly (many of them are hundreds of years old, and some we simply don't know exist)… It's a real challenge.