▁was ▁the ▁main ▁cause , ▁and ▁that , ▁because ▁of ▁this , ▁extra ▁fr iction ▁ar ising ▁out ▁of ▁stri ations ▁( scr atch es ) ▁on ▁a ▁newly ▁installed ▁set ▁of ▁points ▁had ▁allowed ▁the ▁leading ▁wheel ▁of ▁the ▁last ▁carriage ▁to ▁clim b ▁the ▁rail ▁and ▁so ▁der ail . ▁The ▁track ▁geometry ▁at ▁the ▁der ail ment ▁site ▁is ▁a ▁very ▁tight ▁b end ▁and ▁tight ▁tunnel ▁bore , ▁which ▁pre cludes ▁the ▁normal ▁solution ▁for ▁this ▁sort ▁of ▁geometry ▁of ▁cant ing ▁the ▁track ▁by ▁raising ▁the ▁height ▁of ▁one ▁rail ▁relative ▁to ▁the ▁other . ▁ ▁On ▁ 7 ▁July ▁ 2 0 0 5 ▁a ▁defect ive ▁train ▁on ▁the ▁Northern ▁line ▁( ca using ▁its ▁subsequent ▁susp ension ) ▁saved ▁a ▁Northern ▁line ▁train ▁from ▁being ▁blow n ▁up ▁as ▁part ▁of ▁a ▁terror ist ▁attack ▁on ▁the ▁London ▁Under ground ▁and ▁bus ▁systems . ▁Three ▁trains ▁on ▁the ▁Circle ▁and ▁Pic c ad illy ▁lines ▁were ▁blow n ▁up . ▁The ▁Northern ▁line ▁bom ber - to - be ▁instead ▁board ed ▁a ▁bus , ▁which ▁he ▁later ▁ble w ▁up . ▁ ▁On ▁ 1 3 ▁October ▁ 2 0 0 5 ▁the ▁Northern ▁line ▁service ▁was ▁susp ended ▁due ▁to ▁maintenance ▁problems ▁with ▁the ▁emer gency ▁bra king ▁system ▁on ▁the ▁entire ▁train ▁fleet . ▁A ▁series ▁of ▁rail ▁replacement ▁bus es ▁was ▁used ▁to ▁connect ▁out lying ▁stations ▁with ▁other ▁Under ground ▁lines . ▁Full ▁service ▁was ▁restored ▁on ▁ 1 8