They were also the first dual-voltage units to be built , capable of drawing power via 25 kV AC overhead , or 750 V DC third-rail , and the first units in Britain to have fully automatic couplers which allowed both physical coupling and also the connection of control electric and air supplies to be carried out without the need to leave the cab . They were also the first British Rail units fitted with both a pantograph for 25 kilovolt AC overhead lines and shoegear for 750 volt DC third rail supply , and the first units in Britain to have Multi-Function couplers , which allow both physical coupling and also the connection of control electric and air supplies to be carried out without the need to leave the cab .