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It was extended south by one stop to Embankment in 1914 to form an interchange with the Bakerloo and District lines.
In 1913 the Underground Electric Railways Company of London (UERL), owner of the CCE&HR, took over the C&SLR, although they remained separate companies.
With the exception of Morden and Clapham South, where more land was available, the new stations were built on confined corner sites at main road junctions in areas that had been already developed.
Holden made good use of this limited space and designed impressive buildings.
The High Barnet branch has an additional single-station spur at Finchley Central with a shuttle train to Mill Hill East.
Five new stations were built to pavilion-style designs by Stanley Heaps, head of the Underground's Architects Office, stimulating the rapid northward expansion of suburban developments in the following years.
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.
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.There were about 294 million passenger journeys recorded in 2016/17 on the Northern line, making it the busiest on the Underground.