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 50Hz for problems at the motors and interferences with the near telegraph lines, was the beginning 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 AC at , 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
kV50
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 20kV but converted to 25kV in 1953. The 25kV system was then adopted as standard in France, but since substantial amounts of mileage south of Paris had already been electrified at 1.5kV
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. It was possible to use AC motors (and some railways did, with varying success), but they have had less than ideal characteristics for traction purposes. This is because control of speed is difficult without varying the frequency and reliance on voltage to control speed gives a torque at any given speed that is not ideal. This is why DC series motors were the most common choice for traction purposes until the 1990s, as they can be controlled by voltage, and have an almost ideal torque vs speed characteristic. In the 1990s, high-speed trains began to use lighter, lower-maintenance
three-phase AC induction motors. The
N700 Shinkansen uses a three-level converter to convert single-phase AC to AC (via transformer) to DC (via phase-controlled rectifier with thyristor) to a maximum three-phase AC (via a
variable voltage, variable frequency inverter using
IGBTs with
pulse-width modulation) to run the motors. The system works in reverse for
regenerative braking. The choice of was related to the efficiency of power transmission as a function of voltage and cost, not based on a neat and tidy ratio of the supply voltage. For a given power level, a higher voltage allows for a lower current and usually better efficiency at the greater cost for high-voltage equipment. It was found that was an optimal point, where a higher voltage would still improve efficiency but not by a significant amount in relation to the higher costs incurred by the need for larger insulators and greater clearance from structures. To avoid
short circuits, the high voltage must be protected from moisture. Weather events, such as "
the wrong type of snow", have caused failures in the past. An example of atmospheric causes occurred in December 2009, when
four Eurostar trains broke down inside the Channel Tunnel. ==Distribution==