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.

Many trial sites were developed at the beginning of the 20th century but developing a main frequency electrification was not easy. One of those trials, the Seebach-Wettingen railway electrification trial, not successful at 50 Hz for problems at the motors and interferences with the near telegraph lines, was the begin of the standardization of the low frequency railway electrification.

The first successful operational and regular use of a utility frequency 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 1936 by the Deutsche Reichsbahn who electrified part of the Höllentalbahn between Freiburg and Neustadt installing a 20 kV 50 Hz AC system. This part of Germany was in the French zone of occupation after 1945. As a result of examining the German system in 1951 the SNCF electrified the line between Aix-les-Bains and La Roche-sur-Foron in southern France, initially at the same 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 1.5 kV DC, SNCF also continued some major new DC electrification projects, until dual-voltage locomotives were developed in the 1960s.

The main reason why electrification using utility frequency had not been widely adopted before was the lack of reliability of Mercury arc rectifiers that could fit on the train. This in turn related to the requirement to use DC series motors, which required the current to be converted from AC to DC and for that a rectifier is needed. Until the early 1950s, mercury-arc rectifiers were difficult to operate even in ideal conditions and were therefore unsuitable for use in railway locomotives.