, ▁it ▁was ▁served ▁by ▁ 1 9 0 6 ▁Stock . ▁These ▁were ▁replaced ▁by ▁ 1 9 3 8 ▁stock ▁as ▁part ▁of ▁the ▁New ▁Works ▁Program me , ▁later ▁supp lement ed ▁with ▁identical ▁ 1 9 4 9 ▁Stock . ▁When ▁the ▁Pic c ad illy ▁line ▁was ▁extended ▁to ▁He ath row ▁Airport ▁in ▁the ▁ 1 9 7 0 s , ▁its ▁ 1 9 5 9 ▁Stock ▁and ▁ 1 9 5 6 ▁Stock ▁( pro tot ypes ▁of ▁the ▁ 1 9 5 9 ▁Stock ) ▁trains ▁were ▁transferred ▁to ▁the ▁Northern ▁line . ▁As ▁there ▁were ▁not ▁enough ▁ 1 9 5 6 ▁and ▁ 1 9 5 9 ▁Stock ▁trains ▁to ▁replace ▁the ▁Northern ▁line ' s ▁ 1 9 3 8 ▁Stock ▁fleet , ▁they ▁were ▁supp lement ed ▁with ▁newly ▁built ▁ 1 9 7 2 ▁Mark ▁ 1 ▁Stock ▁trains , ▁which ▁all ▁served ▁the ▁line ▁at ▁the ▁same ▁time . ▁A ▁few ▁ 1 9 7 2 ▁Mark ▁ 2 ▁stock ▁trains ▁also ▁ran ▁on ▁this ▁line ▁until ▁going ▁to ▁the ▁J ub ile e ▁and ▁now ▁the ▁Baker lo o ▁where ▁they ▁remain ▁in ▁service . ▁The ▁few ▁ 1 9 5 6 ▁Stock ▁trains ▁were ▁briefly ▁replaced ▁by ▁ 1 9 6 2 ▁Stock ▁transferred ▁from ▁the ▁Central ▁line ▁in ▁ 1 9 9 5 , ▁before ▁the ▁entire ▁Northern ▁line ▁fleet ▁was ▁replaced ▁with ▁ 1 9 9 5 ▁Stock ▁between ▁ 1 9 9 7