- free ▁transition ▁on ▁the ▁run , ▁the ▁locomot ive ▁is ▁equ ipped ▁with ▁on board ▁voltage ▁detect ors , ▁while ▁the ▁overhead ▁wire ▁is ▁equ ipped ▁with ▁two ▁wooden ▁isol ators ▁and ▁a ▁ ▁length ▁of ▁neutral ▁wire ▁to ▁separate ▁the ▁AC ▁and ▁DC ▁fe eds . ▁The ▁neutral ▁section ▁is ▁connected ▁to ▁the ▁rails ▁which ▁serve ▁as ▁the ▁return ▁conduct or ▁on ▁elect r ified ▁lines . ▁ ▁The ▁transition ▁process ▁requires ▁that ▁the ▁locomot ive ▁should ▁automatically ▁be ▁switched ▁off ▁before ▁it ▁reaches ▁the ▁isol ators ▁and ▁the ▁un power ed ▁overhead ▁wire ▁section , ▁and ▁automatically ▁be ▁restart ed ▁after ▁exit ing ▁from ▁under ▁the ▁un power ed ▁wire . ▁This ▁is ▁done ▁by ▁a ▁pair ▁of ▁track ▁magnet s , ▁one ▁on ▁either ▁side ▁of ▁the ▁neutral ▁overhead ▁wire ▁and ▁spac ed ▁ ▁apart . ▁The ▁two ▁magnet s ▁are ▁mounted ▁with ▁their ▁polar ities ▁revers ed ▁in ▁relation ▁to ▁each ▁other ▁and ▁they ▁activ ate ▁a ▁magnetic ▁relay ▁located ▁behind ▁the ▁cow catch er ▁of ▁the ▁locomot ive ▁to ▁do ▁the ▁switching ▁off ▁and ▁restart ing . ▁ ▁P ant ograph s ▁Trans net ▁Fre ight ▁Rail ▁ins isted ▁that ▁the ▁locomot ive ▁must ▁be ▁designed ▁in ▁such ▁a ▁way ▁that ▁the ▁p ant ograph ▁contact ▁sh oe ▁cent res ▁are ▁directly ▁above ▁the ▁bog ie ▁pivot ▁cent res , ▁as ▁was ▁done ▁on ▁the ▁Class ▁ 7 E ▁and ▁Class ▁ 7 E 2 , ▁Series ▁ 1 ▁and ▁Series ▁ 2 , ▁and ▁again ▁on