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Some of these are independent of the contact system used, so that, for example, 750 V DC may be used with either third rail or overhead lines. DC voltages between 600 V and 750 V are used by most tramways and trolleybus networks, as well as some metro systems as the traction motors accept this voltage without the weight of an on-board transformer. The history of railway electrification dates back to the late 19th century when the first electric tramways were introduced in cities like Berlin, London, and New York City. The first sector of the electric utility industry to be deregulated was the network of high-voltage transmission wires, which were designed to make bulk power transfers, over relatively short distances, from large power-generating plants to the cities and towns where the power was needed. A week-long blackout in New York City, calls for "voluntary" conservation, the shutting off of power to large industrial enterprises, and lowering of voltages across the nation, were all evidence of the wreckage that has been made of this most critical infrastructure.

Railway electrification is the development of powering trains and locomotives using electricity instead of diesel or steam power. Different systems are used for urban and intercity areas; some electric locomotives can switch to different supply voltages to allow flexibility in operation. First are the needle telegraphs, in which electric current sent down the telegraph line produces electromagnetic force to move a needle-shaped pointer into position over a printed list. It also suffers during bad weather, the smallest amount of ice or snow rendering top contact third rail systems almost unworkable unless expensive remedies are carried out. There are many other voltage systems used for railway electrification systems around the world, and the list of railway electrification systems covers both standard voltage and non-standard voltage systems. It was first published in April 2016 to provide a standard for travel adaptors suitable for the connection of a non-BS 1363 plug, or to a non-BS 1363 socket-outlet. Today, many countries have extensive electrified railway networks with 375000 km of standard lines in the world, including China, India, Japan, France, Germany, and the United Kingdom. The London Underground in England is one of few networks that uses a four-rail system.

The system was also used for suburban electrification in East London and Manchester, now converted to 25 kV AC. It is now only used for the Tyne and Wear Metro. It was formerly used by the Milwaukee Road from Harlowton, Montana, to Seattle, across the Continental Divide and including extensive branch and loop lines in Montana, and by the Delaware, Lackawanna and Western Railroad (now New Jersey Transit, converted to 25 kV AC) in the United States, and the Kolkata suburban railway (Bardhaman Main Line) in India, before it was converted to 25 kV 50 Hz. In the United States, the New York, New Haven and Hartford Railroad was one of the first major railways to be electrified. In Slovakia, there are two narrow-gauge lines in the High Tatras (one a cog railway). The same system was used for Milan's earliest underground line, Milan Metro's line 1, whose more recent lines use an overhead catenary or a third rail. 6-7 UIC conducted a case study for the conversion of the Bordeaux-Hendaye railway line (France), currently electrified at 1.5 kV DC, to 9 kV DC and found that the conversion would allow to use less bulky overhead wires (saving €20 million per 100 route-km) and lower the losses (saving 2 GWh per year per 100 route-km; equalling about €150,000 p.a.).

In the 1960s the Soviets experimented with boosting the overhead voltage from 3 to 6 kV. The use of medium-voltage DC electrification (MVDC) would solve some of the issues associated with standard-frequency AC electrification systems, especially possible supply grid load imbalance and the phase separation between the electrified sections powered from different phases, whereas high voltage would make the transmission more efficient. Increasing availability of high-voltage semiconductors may allow the use of higher and more efficient DC voltages that heretofore have only been practical with AC. In April 2022, the Biden Administration streamlined environmental reviews for such projects, what is electric cable and in May 2022 announced competitive grants for them funded by the 2021 Bipartisan Infrastructure Law and 2022 Inflation Reduction Act. Electrification is seen as a more sustainable and environmentally friendly alternative to diesel or steam power and is an important part of many countries' transportation infrastructure. In the 1920s and 1930s, many countries worldwide began to electrify their railways. The early electrification of railways used direct current (DC) power systems, which were limited in terms of the distance they could transmit power. However, in the early 20th century, alternating current (AC) power systems were developed, which allowed for more efficient power transmission over longer distances.