The Tower Subway, the world’s first deep-level tube, was a cable car service that ran for three months under the Thames between Tower Hill and Vine Street built in 1870. Greathead greatly revolutionized the tunneling shield by creating a new circular design that allowed the tunnel to be completed at a fraction of the cost of Brunel’s tunnel. This first success began Greathead’s long career in deep-level tunnel digging. This allowed him to improve on Barlow’s design over the years, making it larger and more efficient.
James Greathead later went on to be the chief engineer of the City & South London Railway that was completed in 1890 and now forms part of the Northern line. He also started work on the Central London Railway before his death. The segment between Shephard’s Bush and Liverpool Street now forms part of the Central Line. The Greathead shield was also used in the construction of the other deep-level Underground lines in London.