 On 6 April 1914 , the CCE & HR ( now a part of the Northern line ) opened a one stop extension south from its terminus at Charing Cross . The extension was constructed to facilitate a better interchange between the BS & WR and CCE & HR . Both lines were owned by the UERL which operated two separate and unconnected stations at the northern end of main line station – Trafalgar Square on the BS & WR and Charing Cross on the CCE & HR ( both now part of a combined Charing Cross station ) . The CCE & HR extension was constructed as a single track tunnel running south from Charing Cross as a loop under the River Thames and back . A single platform was constructed on the northbound return section of the loop , and escalators were installed between both sets of deep @-@ level platforms and the sub @-@ surface station . The interchange time was reduced from three minutes fifteen seconds to one minute and forty @-@ five seconds . 
