 In the 1920s , as part of the construction of what is now the Northern line , the CCE & HR was extended south to Waterloo and Kennington where it was connected to the City & South London Railway . The loop tunnel under the river was abandoned ( although the present northbound Northern line platform follows its course ) and two new tunnels were bored south . To this day the southbound Northern line platform is the only one of the four deep level platforms that is not connected to any of the others by deep level walkways . The new extension was opened on 13 September 1926 . 
