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 60Hz), 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.