page_title: 25 kV AC railway electrification
section_title: 2 x 25 kV autotransformer system
hierarchical_section_title: 25 kV AC railway electrification / Variations / 2 x 25 kV autotransformer system
caption_attribution_description: Français&#160;: Photo de la caténaire 25 kV/50 Hz sur la ligne Paris-Cherbourg Auteur&#160;: Bras Cyril Site&#160;: http://www.czarby.com
context_page_description: Railway electrification systems using alternating current at 25 kilovolts are used worldwide, especially for high-speed rail.
context_section_description: The 2 × 25 kV autotransformer system is a split-phase electric power system which supplies 25 kV power to the trains, but transmits power at 50 kV to reduce energy losses.  It should not be confused with the 50 kV system. In this system, the current is mainly carried between the overhead line and a feeder transmission line instead of the rail. The overhead line (3) and feeder (5) are on opposite phases, so the voltage between them is 50 kV, but the voltage between the overhead line (3) and the running rails (4) remains at 25 kV.  Periodic autotransformers (9) divert the return current from the neutral rail, step it up, and send it along the feeder line.  This system is used by Indian Railways, Russian Railways, Italian High Speed Railways, UK High Speed-1 and Crossrail, with some parts of older lines being gradually converted, French lines (LGV lines and some other lines), most Spanish high-speed rail lines, Amtrak and some of the Finnish and Hungarian lines.
caption: 2 × 25 kV overhead line system in France between Paris and Caen
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