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The second connection linked the CCE&HR's Embankment and C&SLR's Kennington stations and provided a new intermediate station at Waterloo to connect to the main line station there and the Bakerloo line.
The smaller-diameter tunnels of the C&SLR were expanded to match the standard diameter of the CCE&HR and the other deep tube lines.
The railway opened in November 1890 from Stockwell to a now-disused station at King William Street.
This was inconveniently placed and unable to cope with the company's traffic so, in 1900, a new route to Moorgate via Bank was opened.
The line then runs under the wide station forecourt and public road outside the station, to the depot.
The engineering of the Morden extension of the C&SLR from Clapham Common to Morden was more demanding, running in tunnels to a point just north of Morden station, which was constructed in a cutting.There were about 294 million passenger journeys recorded in 2016/17 on the Northern line, making it the busiest on the Underground.It is unique in having two different routes through central London and two northern branches.The line has a complicated history, and the current complex arrangement of two main northern branches, two central branches and the southern route reflects its genesis as three separate railways, combined in the 1920s and 1930s.An extension in the 1920s used a route originally planned by a fourth company.