1.
Light rail
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Light rail, light rail transit or fast tram is urban public transport using rolling stock similar to a tramway, but operating at a higher capacity, and often on an exclusive right-of-way. A few light rail networks tend to have closer to rapid transit or even commuter rail. Other light rail networks are tram-like in nature and partially operate on streets, Light rail systems are found throughout the world, on all inhabited continents. They have been popular in recent years due to their lower capital costs. The term light rail was coined in 1972 by the U. S, Urban Mass Transportation Administration to describe new streetcar transformations that were taking place in Europe and the United States. In Germany the term Stadtbahn was used to describe the concept, and many in UMTA wanted to adopt the direct translation, however, UMTA finally adopted the term light rail instead. Light in this context is used in the sense of intended for light loads and fast movement, the infrastructure investment is also usually lighter than would be found for a heavy rail system. The Transportation Research Board defined light rail in 1977 as a mode of urban transportation utilizing predominantly reserved, electrically propelled rail vehicles operate singly or in trains. LRT provides a range of passenger capabilities and performance characteristics at moderate costs. Light rail is a generic international English phrase for these types of rail systems, the use of the generic term light rail avoids some serious incompatibilities between British and American English. The word trolley is used as a synonym for streetcar in the United States. A further difference arose because, while Britain abandoned all of its trams except Blackpool after World War II, when these cities upgraded to new technology, they called it light rail to differentiate it from their existing streetcars since some continued to operate both the old and new systems. Conventional rail technologies including high-speed, freight, commuter/regional, and metro/subway/elevated urban transit systems are considered heavy rail, people movers and personal rapid transit are even lighter, at least in terms of capacity. Monorail is a technology that has been more successful in specialized services than in a commuter transit role. Due to varying definitions, it is hard to distinguish between what is called light rail, and other forms of urban and commuter rail, a system described as light rail in one city may be considered to be a streetcar or tram system in another. Conversely, some lines that are called light rail are in very similar to rapid transit, in recent years. Some light rail systems, such as Sprinter, bear little similarity to urban rail, in the United States, light rail has become a catch-all term to describe a wide variety of passenger rail systems. There is a significant difference in cost between these different classes of light rail transit, tram-like systems are often less expensive than metro-like systems by a factor of two or more
2.
Tram
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A tram is a rail vehicle which runs on tracks along public urban streets, and also sometimes on a segregated right of way. The lines or networks operated by tramcars are called tramways, Tramways powered by electricity, the most common type historically, were once called electric street railways. However, trams were used in urban areas before the universal adoption of electrification. Tram lines may run between cities and/or towns, and/or partially grade-separated even in the cities. Very occasionally, trams also carry freight, Tram vehicles are usually lighter and shorter than conventional trains and rapid transit trains, but the size of trams is rapidly increasing. Some trams may also run on railway tracks, a tramway may be upgraded to a light rail or a rapid transit line. For all these reasons, the differences between the modes of rail transportation are often indistinct. In the United States, the tram has sometimes been used for rubber-tired trackless trains. Today, most trams use electrical power, usually fed by a pantograph, in some cases by a sliding shoe on a third rail. If necessary, they may have dual power systems — electricity in city streets, trams are now included in the wider term light rail, which also includes segregated systems. The English terms tram and tramway are derived from the Scots word tram, referring respectively to a type of truck used in coal mines and the tracks on which they ran. The word tram probably derived from Middle Flemish trame, a Romanesque word meaning the beam or shaft of a barrow or sledge, the identical word la trame with the meaning crossbeam is also used in the French language. The word Tram-car is attested from 1873, although the terms tram and tramway have been adopted by many languages, they are not used universally in English, North Americans prefer streetcar, trolley, or trolleycar. The term streetcar is first recorded in 1840, and originally referred to horsecars, when electrification came, Americans began to speak of trolleycars or later, trolleys. The troller design frequently fell off the wires, and was replaced by other more reliable devices. The terms trolley pole and trolley wheel both derive from the troller, Modern trams often have an overhead pantograph mechanical linkage to connect to power, abandoning the trolley pole altogether. Conventional diesel tourist buses decorated to look like streetcars are sometimes called trolleys in the US, the term may also apply to an aerial ropeway, e. g. the Roosevelt Island Tramway. Over time, the trolley has fallen into informal use
3.
Muni Metro
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Muni Metro is a light rail/streetcar hybrid system serving San Francisco, California, operated by the San Francisco Municipal Railway, a division of the San Francisco Municipal Transportation Agency. With an average ridership of 128,500 passengers as of the fourth quarter of 2014. Muni Metro operates a fleet of 151 light rail vehicles made by Breda, Muni Metro is the modern incarnation of the traditional streetcar system that had served San Francisco since the late 19th century. Recently, the system has undergone expansion, most notably the Third Street Light Rail Project, completed in 2007, other projects, such as the Central Subway, are underway. Muni Metro descended from the traditional streetcar system started on December 28,1912. The first streetcar line, the A Geary, ran from Kearny and Market Streets in the Financial District to Fulton Street, the system slowly expanded, opening the Twin Peaks Tunnel in 1917, allowing streetcars to run to the southwestern quadrant of the city. By 1921, the city was operating 304 miles of trolley lines and 25 miles of cable car lines. The last line to service before 2007 was the N Judah. However, five heavily used streetcar lines traveled for at least part of their routes through tunnels or otherwise reserved right-of-way, as a result, these lines, running PCC streetcars, continued in operation. The new tunnel would be connected to the existing Twin Peaks Tunnel, the new underground stations would feature high platforms, and the older stations would be retrofitted with the same, which meant that the PCCs could not be used in them. Hence, a fleet of new rail vehicles was ordered from Boeing-Vertol. The K and M lines were extended to Balboa Park during this time, on February 18,1980, the Muni Metro was officially inaugurated, with weekday N line service in the subway. The Metro service was implemented in phases, and the subway was served only on weekdays until 1982. The K Ingleside line began using the Metro subway on weekdays on June 11,1980, the L Taraval and M Ocean View lines on December 17,1980, and lastly the J Church line on June 17,1981. Meanwhile, weekend service on all five lines continued to use PCC cars operating on the surface of Market Street through to the Transbay Terminal, finally, on November 20,1982, the Muni Metro subway began operating seven days a week. At the time, there were no plans to revive any service on the surface of Market Street or return PCCs to regular running. However, tracks were rehabilitated for the 1983 Historic Trolley Festival, Muni criticism had been something of a feature of life in San Francisco, and not without reason. The Boeing trains were sub-par and grew crowded quickly, Muni did take steps to address these problems
4.
San Francisco
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San Francisco, officially the City and County of San Francisco, is the cultural, commercial, and financial center of Northern California. It is the birthplace of the United Nations, the California Gold Rush of 1849 brought rapid growth, making it the largest city on the West Coast at the time. San Francisco became a consolidated city-county in 1856, after three-quarters of the city was destroyed by the 1906 earthquake and fire, San Francisco was quickly rebuilt, hosting the Panama-Pacific International Exposition nine years later. In World War II, San Francisco was a port of embarkation for service members shipping out to the Pacific Theater. Politically, the city votes strongly along liberal Democratic Party lines, San Francisco is also the headquarters of five major banking institutions and various other companies such as Levi Strauss & Co. Dolby, Airbnb, Weebly, Pacific Gas and Electric Company, Yelp, Pinterest, Twitter, Uber, Lyft, Mozilla, Wikimedia Foundation, as of 2016, San Francisco is ranked high on world liveability rankings. The earliest archaeological evidence of habitation of the territory of the city of San Francisco dates to 3000 BC. Upon independence from Spain in 1821, the became part of Mexico. Under Mexican rule, the system gradually ended, and its lands became privatized. In 1835, Englishman William Richardson erected the first independent homestead, together with Alcalde Francisco de Haro, he laid out a street plan for the expanded settlement, and the town, named Yerba Buena, began to attract American settlers. Commodore John D. Sloat claimed California for the United States on July 7,1846, during the Mexican–American War, montgomery arrived to claim Yerba Buena two days later. Yerba Buena was renamed San Francisco on January 30 of the next year, despite its attractive location as a port and naval base, San Francisco was still a small settlement with inhospitable geography. The California Gold Rush brought a flood of treasure seekers, with their sourdough bread in tow, prospectors accumulated in San Francisco over rival Benicia, raising the population from 1,000 in 1848 to 25,000 by December 1849. The promise of fabulous riches was so strong that crews on arriving vessels deserted and rushed off to the gold fields, leaving behind a forest of masts in San Francisco harbor. Some of these approximately 500 abandoned ships were used at times as storeships, saloons and hotels, many were left to rot, by 1851 the harbor was extended out into the bay by wharves while buildings were erected on piles among the ships. By 1870 Yerba Buena Cove had been filled to create new land, buried ships are occasionally exposed when foundations are dug for new buildings. California was quickly granted statehood in 1850 and the U. S. military built Fort Point at the Golden Gate, silver discoveries, including the Comstock Lode in Nevada in 1859, further drove rapid population growth. With hordes of fortune seekers streaming through the city, lawlessness was common, and the Barbary Coast section of town gained notoriety as a haven for criminals, prostitution, entrepreneurs sought to capitalize on the wealth generated by the Gold Rush
5.
California
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California is the most populous state in the United States and the third most extensive by area. Located on the western coast of the U. S, California is bordered by the other U. S. states of Oregon, Nevada, and Arizona and shares an international border with the Mexican state of Baja California. Los Angeles is Californias most populous city, and the second largest after New York City. The Los Angeles Area and the San Francisco Bay Area are the nations second- and fifth-most populous urban regions, California also has the nations most populous county, Los Angeles County, and its largest county by area, San Bernardino County. The Central Valley, an agricultural area, dominates the states center. What is now California was first settled by various Native American tribes before being explored by a number of European expeditions during the 16th and 17th centuries, the Spanish Empire then claimed it as part of Alta California in their New Spain colony. The area became a part of Mexico in 1821 following its war for independence. The western portion of Alta California then was organized as the State of California, the California Gold Rush starting in 1848 led to dramatic social and demographic changes, with large-scale emigration from the east and abroad with an accompanying economic boom. If it were a country, California would be the 6th largest economy in the world, fifty-eight percent of the states economy is centered on finance, government, real estate services, technology, and professional, scientific and technical business services. Although it accounts for only 1.5 percent of the states economy, the story of Calafia is recorded in a 1510 work The Adventures of Esplandián, written as a sequel to Amadis de Gaula by Spanish adventure writer Garci Rodríguez de Montalvo. The kingdom of Queen Calafia, according to Montalvo, was said to be a land inhabited by griffins and other strange beasts. This conventional wisdom that California was an island, with maps drawn to reflect this belief, shortened forms of the states name include CA, Cal. Calif. and US-CA. Settled by successive waves of arrivals during the last 10,000 years, various estimates of the native population range from 100,000 to 300,000. The Indigenous peoples of California included more than 70 distinct groups of Native Americans, ranging from large, settled populations living on the coast to groups in the interior. California groups also were diverse in their organization with bands, tribes, villages. Trade, intermarriage and military alliances fostered many social and economic relationships among the diverse groups, the first European effort to explore the coast as far north as the Russian River was a Spanish sailing expedition, led by Portuguese captain Juan Rodríguez Cabrillo, in 1542. Some 37 years later English explorer Francis Drake also explored and claimed a portion of the California coast in 1579. Spanish traders made unintended visits with the Manila galleons on their trips from the Philippines beginning in 1565
6.
San Francisco 4th and King Street Station
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It is next to a Muni Metro light rail station, which provides connections to downtown San Francisco and Bay Area Rapid Transit. The station is in the Mission Bay/China Basin area, bordered on the north by Townsend Street, east by 3rd Street, west by 4th Street and it opened on June 21,1975, replacing a station built in 1914 at 3rd and Townsend, one block away. The Muni extension to the station was opened in 1998, the Caltrain Downtown Extension project to the rebuilt Transbay Terminal includes the construction of an underground 4th and King station. The underground station will be next to the current station on the Townsend side, until that time, California High-Speed Rail trains may also utilize the existing station. All 96 weekday trains and all Saturday and Sunday trains utilize the station as a terminal, San Francisco Giants service 4th and King is one block from AT&T Park, the home of the San Francisco Giants. Caltrain usually operates special trains to transport fans to baseball games, special trains usually depart about fifteen minutes after the last out. One special train, leaving 15 minutes after the last out, expresses to San Carlos and this train is not available for daytime weekday games. Giants fans can either walk or take the Muni to the ballpark, Muni operates special trains from Caltrain to the ballpark on game days. 4th and King hosts a number of Muni bus lines, the E Embarcadero historic streetcar line, and Munis T Third Street and N Judah lines run to Market St downtown. N Judah service replaced the J Church on June 30,2007, the nearest BART access is the Powell Street station, a 1-mile walk up 4th street then left on Market St. California Shuttle Bus runs to Los Angeles via Oakland and San Jose, service to Chinatown via Munis Central Subway will connect to this station in 2019 after a realignment of the T Third Street lines route
7.
San Francisco Municipal Railway
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The San Francisco Municipal Railway is the public transit system for the city and county of San Francisco, California. In 2006, it served 46.7 square miles with an budget of about $700 million. In ridership Muni is the seventh largest transit system in the United States, with 210,848,310 rides in 2006 and the second largest in California behind Metro in Los Angeles. With a fleet average speed of 8.1 mph, it is the slowest major urban transit system in America and one of the most expensive to operate, costing $19.21 per mile per bus and $24.37 per mile per train. However, it has more boardings per mile and more vehicles in operation than similar transit agencies, many weekday riders are commuters, as the daytime weekday population in San Francisco exceeds its normal residential population. Muni shares four metro stations with BART, on weekends, most Muni bus lines are scheduled to run every ten to twenty minutes. However, complaints of unreliability, especially on less-often-served lines and older lines, are a system-wide problem. Muni has had difficulty meeting a stated goal of 85% voter-demanded on-time service. Most intercity connections are provided by BART and Caltrain heavy rail, AC Transit buses at the Transbay Terminal, 70% of stops are spaced closer than recommended range of 800–1,000 feet apart. Muni is short for the Municipal in San Francisco Municipal Railway and is not an acronym, thus, the Muni metro is often called the train or the streetcar. Most San Francisco natives use Muni when speaking about the system in general, the E Embarcadero and F Market & Wharves lines are referred to by Muni as a historic streetcar line rather than as a heritage railway. Munis logo is a stylized, trademarked worm version of the word muni and this logo was designed by San Francisco-based graphic designer Walter Landor in the mid-1970s. Bus and trolleybus lines have number designations, rail lines have letters, except for cable cars, cash fares are $2.50 for adults, $1. Clipper card fares are $2.25 for adults and $1 for seniors, youth, proof-of-payment, which fare inspectors may demand at any time, is either a Clipper card, Muni Passport, or paper transfer. One fare entitles a rider to unlimited vehicle transfers for the next 90 to 120 minutes, cable cars are $7 one way, with no transfers unless the rider has a Muni Passport or Fast Pass. As of September 2014 monthly passes cost $70 for adults, $35 for low-income residents, or $24 for youth, seniors, passes are valid on all Muni lines—including cable cars—and the $83 adult Fast Pass allows BART transit entirely within San Francisco. Other passes and stickers are valid on all Muni lines, including cable cars, cable car fare is $7 per trip, with no transfers issued or accepted. Muni has implemented a smart card payment system known as Clipper
8.
Hitachi Rail Italy
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Hitachi Rail Italy S. p. A. is a rail transport engineering company based in Italy whose main products are designing and manufacturing of railway and mass transit vehicles. A subsidiary of Finmeccanica, the company was sold to Hitachi in February 2015, after the deal was finalized, the current name was adapted in November 2015 to reflect the new ownership. Ansaldo & C. was registered in Genoa as a manufacturer of locomotives, rail rolling stock. The company was supported by the Minister of Finance, Camillo Benso, Count of Cavour, in 1886, Ernesto Breda founded Ing. Ernesto Breda and C. the company which became Società Italiana Ernesto Breda in 1899, in 1989 both Ansaldo and Breda participated in a consortium for the design and manufacturing of the ETR500 high speed train. AnsaldoBreda was formed in 2001 by the merger of Ansaldo Trasporti and Breda Costruzioni Ferroviarie and it has production sites at four locations in Italy, Naples, Reggio Calabria, Palermo, and Pistoia. The company employs about 2,400 people, salvatore Bianconi was the Chief Executive Officer of AnsaldoBreda from 2008 to 2011, when he was succeeded by Maurizio Manfellotto. On 2 November 2015 AnsaldoBreda was sold and changed its name to Hitachi Rail Italy, Ansaldo entered the age of the steam locomotive in 1854 with its model FS113, also known as Sampierdarena. In 1908 SIEBs thousandth locomotive was built, a model FS685 with serial number 600 now preserved in the Leonardo da Vinci Museum of Science, in 1936 Breda entered the electric locomotive era with the production of the FS Class ETR200 series electric multiple unit. In 1939 this type set the speed record for rail vehicles at 203 km/h. Although only 18 trains of type were constructed they remained in service for a long time until they were finally withdrawn from service in 1993. In 1976 the FS ETR400 entered service, fiat Ferroviaria manufactured the body and bogies, whilst Ansaldo produced the power unit. It was the first train in the world featuring active body tilting to enter service and was capable of speeds of up to 250 km/h. The TREVI Consortium, of which Ansaldo and Breda Costruzioni Ferroviarie were members, during the tests, ETR500 Y2 achieved the current rail speed record in Turkey, reaching 303 km/h. The train was capable of exceeding 300 km/h and it reached 362 km/h in the Monte Bibele tunnel between Florence and Bologna in 2009, setting a speed record for trains in a tunnel. Another high speed train Frecciarossa 1000 high speed train was developed in a consortium with Bombardier Transportation, the first ETR1000 was christened Mennea in honour of the Italian athlete Pietro Mennea who had died five days earlier. Besides the construction of locomotives, AnsaldoBreda manufactures railway cars, trams, and trains for rail, high-speed rail. AnsaldoBreda has also developed a driverless multiple unit train, since acquired by Hitachi, production of some British Rail Class 802 has been shifted to Hitachi Rail Italys Pistoia plant due to Hitachi Rails Newton Aycliffe, England plant being at capacity
9.
Siemens
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Siemens AG is a German conglomerate company headquartered in Berlin and Munich and the largest manufacturing and electronics company in Europe with branch offices abroad. The principal divisions of the company are Industry, Energy, Healthcare, and Infrastructure & Cities, the company is a component of the Euro Stoxx 50 stock market index. Siemens and its subsidiaries employ approximately 362,000 people worldwide, the von Siemens family remain the largest shareholder with a stake of 6. 9% percent. Siemens & Halske was founded by Werner von Siemens and Johann Georg Halske on 12 October 1847, based on the telegraph, their invention used a needle to point to the sequence of letters, instead of using Morse code. The company, then called Telegraphen-Bauanstalt von Siemens & Halske, opened its first workshop on 12 October, in 1848, the company built the first long-distance telegraph line in Europe,500 km from Berlin to Frankfurt am Main. In 1850, the younger brother, Carl Wilhelm Siemens, later Sir William Siemens. The London agency became an office in 1858. In the 1850s, the company was involved in building long distance telegraph networks in Russia, in 1855, a company branch headed by another brother, Carl Heinrich von Siemens, opened in St Petersburg, Russia. In 1867, Siemens completed the monumental Indo-European telegraph line, in 1867, Werner von Siemens described a dynamo without permanent magnets. A similar system was independently invented by Charles Wheatstone. In 1881, a Siemens AC Alternator driven by a watermill was used to power the worlds first electric lighting in the town of Godalming. The company continued to grow and diversified into electric trains and light bulbs, in 1890, the founder retired and left running the company to his brother Carl and sons Arnold and Wilhelm. In 1887, it opened its first office in Japan, Siemens & Halske was incorporated in 1897, and then merged parts of its activities with Schuckert & Co. Nuremberg in 1903 to become Siemens-Schuckert, in 1907, Siemens had 34,324 employees and was the seventh-largest company in the German empire by number of employees. In 1919, S & H and two companies jointly formed the Osram lightbulb company. During the 1920s and 1930s, S & H started to manufacture radios, television sets, in the 1920s, Siemens constructed the Ardnacrusha Hydro Power station on the River Shannon in the then Irish Free State, and it was a world first for its design. The company is remembered for its desire to raise the wages of its workers only to be overruled by the Cumann na nGaedheal government. Siemens exploited the forced labour of deported people in extermination camps, the company owned a plant in Auschwitz concentration camp
10.
Track gauge
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In rail transport, track gauge is the spacing of the rails on a railway track and is measured between the inner faces of the load-bearing rails. All vehicles on a network must have running gear that is compatible with the track gauge, as the dominant parameter determining interoperability, it is still frequently used as a descriptor of a route or network. There is a distinction between the gauge and actual gauge at some locality, due to divergence of track components from the nominal. Railway engineers use a device, like a caliper, to measure the actual gauge, the nominal track gauge is the distance between the inner faces of the rails. In current practice, it is specified at a distance below the rail head as the inner faces of the rail head are not necessarily vertical. In some cases in the earliest days of railways, the company saw itself as an infrastructure provider only. Colloquially the wagons might be referred to as four-foot gauge wagons, say and this nominal value does not equate to the flange spacing, as some freedom is allowed for. An infrastructure manager might specify new or replacement track components at a variation from the nominal gauge for pragmatic reasons. Track is defined in old Imperial units or in universally accepted metric units or SI units, Imperial units were established in United Kingdom by The Weights and Measures Act of 1824. In addition, there are constraints, such as the load-carrying capacity of axles. Narrow gauge railways usually cost less to build because they are lighter in construction, using smaller cars and locomotives, as well as smaller bridges, smaller tunnels. Narrow gauge is often used in mountainous terrain, where the savings in civil engineering work can be substantial. Broader gauge railways are generally expensive to build and require wider curves. There is no single perfect gauge, because different environments and economic considerations come into play, a narrow gauge is superior if ones main considerations are economy and tight curvature. For direct, unimpeded routes with high traffic, a broad gauge may be preferable, the Standard, Russian, and 46 gauges are designed to strike a reasonable balance between these factors. In addition to the general trade-off, another important factor is standardization, once a standard has been chosen, and equipment, infrastructure, and training calibrated to that standard, conversion becomes difficult and expensive. This also makes it easier to adopt an existing standard than to invent a new one and this is true of many technologies, including railroad gauges. The reduced cost, greater efficiency, and greater economic opportunity offered by the use of a common standard explains why a number of gauges predominate worldwide
11.
Railway electrification system
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A railway electrification system supplies electric power to railway trains and trams without an on-board prime mover or local fuel supply. Electrification has many advantages but requires significant capital expenditure, selection of an electrification system is based on economics of energy supply, maintenance, and capital cost compared to the revenue obtained for freight and passenger traffic. Different systems are used for urban and intercity areas, some electric locomotives can switch to different supply voltages to allow flexibility in operation, Electric railways use electric locomotives to haul passengers or freight in separate cars or electric multiple units, passenger cars with their own motors. Electricity is typically generated in large and relatively efficient generating stations, transmitted to the railway network, some electric railways have their own dedicated generating stations and transmission lines but most purchase power from an electric utility. The railway usually provides its own lines, switches and transformers. Power is supplied to moving trains with a continuous conductor running along the track usually takes one of two forms. The first is a line or catenary wire suspended from poles or towers along the track or from structure or tunnel ceilings. Locomotives or multiple units pick up power from the wire with pantographs on their roofs that press a conductive strip against it with a spring or air pressure. Examples are described later in this article, the second is a third rail mounted at track level and contacted by a sliding pickup shoe. Both overhead wire and third-rail systems usually use the rails as the return conductor. In comparison to the alternative, the diesel engine, electric railways offer substantially better energy efficiency, lower emissions. Electric locomotives are usually quieter, more powerful, and more responsive and they have no local emissions, an important advantage in tunnels and urban areas. Different regions may use different supply voltages and frequencies, complicating through service, the limited clearances available under catenaries may preclude efficient double-stack container service. Possible lethal electric current due to risk of contact with high-voltage contact wires, overhead wires are safer than third rails, but they are often considered unsightly. These are independent of the system used, so that. The permissible range of voltages allowed for the voltages is as stated in standards BS EN50163. These take into account the number of trains drawing current and their distance from the substation, railways must operate at variable speeds. Until the mid 1980s this was only practical with the brush-type DC motor, since such conversion was not well developed in the late 19th century and early 20th century, most early electrified railways used DC and many still do, particularly rapid transit and trams
12.
Overhead line
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An overhead line or overhead wire is used to transmit electrical energy to trams, trolleybuses, or trains. Overhead line is designed on the principle of one or more overhead wires situated over rail tracks, the feeder stations are usually fed from a high-voltage electrical grid. Electric trains that collect their current from overhead lines use a device such as a pantograph and it presses against the underside of the lowest overhead wire, the contact wire. Current collectors are electrically conductive and allow current to flow through to the train or tram, non-electric locomotives may pass along these tracks without affecting the overhead line, although there may be difficulties with overhead clearance. Alternative electrical power transmission schemes for trains include third rail, ground-level power supply, batteries and this article does not cover regenerative braking, where the traction motors act as generators to retard movement and return power to the overhead. To achieve good high-speed current collection, it is necessary to keep the wire geometry within defined limits. This is usually achieved by supporting the wire from a second wire known as the messenger wire or catenary. This wire approximates the path of a wire strung between two points, a catenary curve, thus the use of catenary to describe this wire or sometimes the whole system. This wire is attached to the wire at regular intervals by vertical wires known as droppers or drop wires. It is supported regularly at structures, by a pulley, link, the whole system is then subjected to a mechanical tension. As the contact wire makes contact with the pantograph, the insert on top of the pantograph is worn down. The straight wire between supports will cause the wire to cross over the whole surface of the pantograph as the train travels around the curve, causing uniform wear. On straight track, the wire is zigzagged slightly to the left. The movement of the wire across the head of the pantograph is called the sweep. The zigzagging of the line is not required for trolley poles. Depot areas tend to have only a wire and are known as simple equipment or trolley wire. When overhead line systems were first conceived, good current collection was only at low speeds. Compound equipment - uses a second wire, known as the auxiliary
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