The third rail is an alternative to overhead lines that transmit power to trains by means of pantographs attached to the trains. Whereas overhead-wire systems can operate at 25 kV or more, using alternating current (AC), the smaller clearance around a live rail imposes a maximum of about 1200 V, with some systems using 1500 V (Line 4, Guangzhou Metro, Line 5, Guangzhou Metro, Line 3, Shenzhen Metro), and direct current (DC) is used. Trains on some lines or networks use both power supply modes (see below).