25 kV AC railway electrification

25 kV alternating current electrification is commonly used in railway electrification systems worldwide, especially for high-speed rail.

This electrification is ideal for railways that cover long distances or carry heavy traffic. After some experimentation before World War II in Hungary and in the Black Forest in Germany, it came into widespread use in the 1950s.

One of the reasons why it was not introduced earlier was the lack of suitable small and lightweight control and rectification equipment before the development of solid-state rectifiers and related technology. Another reason was the increased clearance distances required where it ran under bridges and in tunnels, which would have required major civil engineering in order to provide the increased clearance to live parts.

Railways using older, lower-capacity direct current systems have introduced or are introducing 25 kV AC instead of 3 kV DC/1.5 kV DC for their new high-speed lines.

The first successful operational and regular use of the 50 Hz system dates back to 1931, tests having run since 1922. It was developed by Kálmán Kandó in Hungary, who used 16 kV AC at 50 Hz, asynchronous traction, and an adjustable number of (motor) poles. The first electrified line for testing was Budapest–Dunakeszi–Alag. The first fully electrified line was Budapest–Győr–Hegyeshalom (part of the Budapest–Vienna line). Although Kandó's solution showed a way for the future, railway operators outside of Hungary showed a lack of interest in the design.

The first railway to use this system was completed in 1951 by SNCF between Aix-les-Bains and La Roche-sur-Foron in southern France, initially at 20 kV but converted to 25 kV in 1953. The 25 kV system was then adopted as standard in France, but since substantial amounts of mileage south of Paris had already been electrified at 1,500 V DC, SNCF also continued some major new DC electrification projects, until dual-voltage locomotives were developed in the 1960s.

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