The cut-and-cover method used to construct London's early underground lines was expensive and disruptive. But to take the underground any deeper, engineers had to overcome three technical barriers. They needed a safe, reliable means of tunnelling through the London clay, a way to move passengers up and down at deep-level stations, and a clean alternative to steam power for the trains.
The pieces eventually came together to launch the first deep-level underground line in 1890-the City & South London Railway. More deep Tube lines followed, though the remaining challenges were not technical but financial. American entrepreneur Charles Tyson Yerkes - who made a fortune running street railways in Chicago - stepped in and made the Underground an attractive proposition for investors.