The world's first underground electric railway, the world's first deep tunnel railway, and the first purpose-built railway tunnel under the Thames. This became what we know as the Bank branch of the Northern Line.
Opened in 1863 the Metropolitan line was actually the world's first underground passenger railway but it was built with the cut-and-cover technique rather than by tunnelling, and the trains were powered by steam rather than clean electricity.
Londonist: Time Machine informed that 1922-4 the tunnels were widened to enable larger rolling stock. When the line was reopened on 1 December 1924 the first train was driven by 17-year old Marian Stanley, the daughter of Lord Ashfield and almost certainly the first woman ever to drive a tube train, and probably the youngest person too.
This image comes from Nick's page all about some CSLR abandoned tunnels and his visit to see them, before the Jubilee Line Extension severed them, so before 1999.
See Londonist for a good succinct history of this line.
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