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Railway electrification systems using alternating current (AC) at 25 kilovolts (kV) are used worldwide, especially for high-speed rail. It is usually supplied at the standard utility frequency (typically 50 or 60 Hz), which simplifies traction substations. The development of 25 kV AC electrification is closely connected with that of successfully using utility frequency.
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 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 required under bridges and in tunnels, which would have required major civil engineering in order to provide the increased clearance to live parts. Where existing loading gauges were more generous, this was less of an issue.
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.