Class 319 units have dual power pick-up , from either 25 kV alternating current ( AC ) overhead lines for services north of London , or 750 V direct current ( DC ) third rail to the south . Class 319 units have dual-power pick-up , from either 25 kV alternating current ( AC ) overhead lines for services north of London , or 750 V direct current ( DC ) third rail to the south .