▁and ▁ 1 9 9 9 . ▁ ▁Today , ▁all ▁Northern ▁line ▁trains ▁consist ▁of ▁ 1 9 9 5 ▁Stock ▁in ▁the ▁Under ground ▁li very ▁of ▁red , ▁white ▁and ▁blue . ▁In ▁common ▁with ▁the ▁other ▁deep - level ▁lines , ▁the ▁trains ▁are ▁the ▁smaller ▁of ▁the ▁two ▁loading ▁gau ges ▁used ▁on ▁the ▁system . ▁ 1 9 9 5 ▁stock ▁has ▁autom ated ▁announ c ements ▁and ▁quick - close ▁doors . ▁If ▁the ▁proposed ▁split ▁of ▁the ▁line ▁takes ▁place ▁( initial ▁estimates ▁of ▁ 2 0 1 8 ▁having ▁been ▁abandoned ▁to ▁focus ▁on ▁completion ▁of ▁the ▁B atter se a ▁and ▁N ine ▁El ms ▁extension ▁work ), ▁ 1 9 ▁new ▁trains ▁will ▁be ▁added ▁to ▁the ▁existing ▁fleet ▁of ▁ 1 0 6 ▁trains , ▁though ▁additional ▁trains ▁beyond ▁the ▁extra ▁ 1 9 ▁trains ▁may ▁be ▁required ▁to ▁provide ▁a ▁full ▁service ▁for ▁the ▁new ▁B atter se a ▁extension . ▁ ▁T unn els ▁Although ▁two ▁other ▁London ▁Under ground ▁lines ▁operate ▁fully ▁under ground , ▁the ▁Northern ▁line ▁is ▁unusual ▁in ▁that ▁it ▁is ▁a ▁deep - level ▁t ube ▁line ▁that ▁serves ▁the ▁outer ▁sub ur bs ▁of ▁South ▁London ▁yet ▁there ▁is ▁only ▁one ▁station ▁above ▁ground ▁( M orden ▁t ube ▁station ) ▁while ▁the ▁rest ▁of ▁this ▁part ▁of ▁the ▁line ▁is ▁deep ▁below ▁ground . ▁The ▁short ▁section ▁to ▁M orden ▁dep ot ▁is ▁also ▁above ▁ground . ▁This ▁is ▁partly ▁because ▁its