▁southern ▁extension ▁into ▁the ▁outer ▁sub ur bs ▁was ▁not ▁done ▁by ▁taking ▁over ▁an ▁existing ▁surface ▁line ▁as ▁was ▁generally ▁the ▁case ▁with ▁routes ▁like ▁the ▁Central , ▁J ub ile e ▁and ▁Pic c ad illy ▁lines . ▁A part ▁from ▁the ▁core ▁central ▁under ground ▁t unn els , ▁part ▁of ▁the ▁section ▁between ▁Hend on ▁and ▁Col ind ale ▁is ▁also ▁under ground . ▁As ▁b icy cles ▁are ▁not ▁allowed ▁in ▁tunnel ▁sections ▁( even ▁if ▁no ▁station ▁is ▁in ▁that ▁section ) ▁as ▁they ▁would ▁h inder ▁ev acu ation , ▁they ▁are ▁limited ▁to ▁High ▁Bar net ▁– ▁East ▁Fin ch ley , ▁the ▁Mill ▁Hill ▁East ▁branch , ▁Ed g ware ▁– ▁Col ind ale ▁and ▁Hend on ▁Central ▁– ▁Gold ers ▁Green . ▁There ▁are ▁also ▁time - based ▁restrictions ▁for ▁the ▁sections ▁where ▁b icy cles ▁are ▁allowed . ▁ ▁The ▁tunnel ▁from ▁M orden ▁to ▁East ▁Fin ch ley ▁via ▁Bank , ▁ 1 7   m iles ▁ 5 2 8   y ards ▁( 2 7 . 8 4 1   km ), ▁was ▁for ▁a ▁time ▁the ▁longest ▁in ▁the ▁world . ▁The ▁Channel ▁T unnel ▁linking ▁the ▁UK ▁and ▁France ▁together ▁is ▁now ▁longer . ▁ ▁Dep ots ▁The ▁Northern ▁line ▁is ▁serv iced ▁by ▁four ▁dep ots . ▁The ▁main ▁one ▁is ▁at ▁Gold ers ▁Green , ▁adjacent ▁to ▁Gold ers ▁Green ▁t ube ▁station , ▁while ▁the ▁second , ▁at ▁M orden , ▁is ▁south ▁of