Three-phase AC railway electrification

Three-phase AC railway electrification, which promised some advantages over established DC electric rail power and steam traction, started at the turn of the twentieth century. The first standard gauge line, from 1899 to 1933, was from Burgdorf to Thun in Switzerland (40 km or 25 mi). Italy was the major user, from 1901 until 1976, although lines through two tunnels also used the system; the Simplon Tunnel between Switzerland and Italy from 1906 to 1930 (but not connected to the Italian system), and the Cascade Tunnel of the Great Northern Railway in the United States from 1909 to 1939. Single phase AC railways with a single overhead line proved more practical.

Since the 1980s, modern electric locomotives use three-phase AC internally, generated from a single overhead line, thanks to advances in semiconductor inverter technology. These inverters are also used in electric cars, from a DC battery, or from DC photovoltaic panels into the three-phase AC grid.