Question: What was the first underground line in the British city holding the summer Olympics in 2012?

full_context: Document 1: London Underground: The system's first tunnels were built just below the surface, using the cut - and - cover method; later, smaller, roughly circular tunnels -- which gave rise to its nickname, the Tube -- were dug through at a deeper level. The system has 270 stations and 250 miles (400 km) of track. Despite its name, only 45% of the system is actually underground in tunnels, with much of the network in the outer environs of London being on the surface. In addition, the Underground does not cover most southern parts of Greater London, with less than 10% of the stations located south of the River Thames.
Document 2: Water polo at the 2012 Summer Olympics – Men's tournament: The men's tournament of water polo at the 2012 Summer Olympics at London, Great Britain, began on 29 July and lasted until 12 August 2012. All games were held at the Water Polo Arena.
Document 3: Great Britain at the 2012 Summer Olympics: Great Britain at the 2012 Summer Olympics Flag of the United Kingdom IOC code GBR NOC British Olympic Association in London Competitors 541 in 26 sports Flag bearer Chris Hoy (opening) Ben Ainslie (closing) Medals Ranked 3rd Gold Silver Bronze Total 29 17 19 65 Summer Olympics appearances (overview) 1896 1900 1904 1908 1912 1920 1924 1928 1932 1936 1948 1952 1956 1960 1964 1968 1972 1976 1980 1984 1988 1992 2000 2008 2012 2016 Other related appearances 1906 Intercalated Games
Document 4: London Underground: The Underground has its origins in the Metropolitan Railway, the world's first underground railway. Opened in 1863, it is now part of the Circle, Hammersmith & City and Metropolitan lines; the first line to operate underground electric traction trains, the City & South London Railway in 1890, is now part of the Northern line. The network has expanded to 11 lines, and in 2016 -- 17 carried 1.379 billion passengers, making it the world's 11th busiest metro system. The 11 lines collectively handle approximately 4.8 million passengers a day.
Document 5: 2020 Summer Olympics: The 2020 Summer Olympics, officially known as the Games of the XXXII Olympiad (Japanese: 第三十二回オリンピック競技大会, Hepburn: Dai Sanjūni - kai Orinpikku Kyōgi Taikai) and commonly known as Tokyo 2020, is a forthcoming international multi-sport event that is scheduled to take place from 24 July to 9 August 2020.
Document 6: 2012 Summer Olympics: The 2012 Summer Olympics, formally the Games of the XXX Olympiad and commonly known as London 2012, was an international multi-sport event that was held from 27 July to 12 August 2012 in London, United Kingdom. The first event, the group stage in women's football, began on 25 July at the Millennium Stadium in Cardiff, followed by the opening ceremonies on 27 July. 10,768 athletes from 204 National Olympic Committees (NOCs) participated.
Document 7: 2012 Summer Paralympics opening ceremony: The 2012 Summer Paralympics opening ceremony was held on 29 August 2012, starting at 20:30 BST and marking the official opening of the 2012 Summer Paralympics in London, England. The show – named Enlightenment – had Jenny Sealey and Bradley Hemmings as its artistic directors, leading a team that included Jon Bausor as set designer and Moritz Junge as costume designer. Queen Elizabeth II officially opened the Games. The ceremony was performed in the Olympic stadium in London in front of a capacity audience of 80,000 people.
Document 8: Railway electrification in Great Britain: The first electric railway in Great Britain was Volk's Electric Railway in Brighton, a pleasure railway, which opened in 1883, still functioning to this day. The London Underground began operating electric services using a fourth rail system in 1890 on the City and South London Railway, now part of the London Underground Northern line. The Liverpool Overhead Railway followed in 1893, being designed from the outset to be electric traction, unlike the City and South London Railway which was designed to be cable hauled initially. Main line electrification of some suburban lines began in the early years of the 20th century, using a variety of different systems. In 1921 a government committee chose 1,500 V DC overhead to be the national standard, but little implementation followed and many different systems co-existed. During the interwar period, the Southern Railway adopted the 660 V DC third rail system as its standard and greatly expanded this system across its network of lines south of London.
Document 9: London Olympics: London hosted the Olympic Games in 1908, 1948 and 2012. The 2012 Summer Olympics made London the first city to have hosted the modern Games of three Olympiads. London is the only city in the United Kingdom to have ever hosted the Olympics; the United States is the only country to have hosted Summer Olympics on more occasions than the UK. Also, London is the only city to have bid more than once and still hold a 100% record.
Document 10: New Delhi: The Delhi Metro is a rapid transit system serving New Delhi, Delhi, Gurgaon, Faridabad, Noida, and Ghaziabad in the National Capital Region of India. Delhi Metro is the world's 12th largest metro system in terms of length. Delhi Metro was India's first modern public transportation system, which had revolutionised travel by providing a fast, reliable, safe, and comfortable means of transport. The network consists of six lines with a total length of 189.63 kilometres (117.83 miles) with 142 stations, of which 35 are underground, five are at-grade, and the remainder are elevated. All stations have escalators, elevators, and tactile tiles to guide the visually impaired from station entrances to trains. It has a combination of elevated, at-grade, and underground lines, and uses both broad gauge and standard gauge rolling stock. Four types of rolling stock are used: Mitsubishi-ROTEM Broad gauge, Bombardier MOVIA, Mitsubishi-ROTEM Standard gauge, and CAF Beasain Standard gauge.
Document 11: 2008 Summer Olympics: The 2008 Summer Olympic Games, officially known as the Games of the XXIX Olympiad (Chinese: 第二十九届夏季奥林匹克运动会; pinyin: Dì Èrshíjiǔ Jiè Xiàjì Àolínpǐkè Yùndònghuì) and commonly known as Beijing 2008, was an international multi-sport event that was held from 8 to 24 August 2008 in Beijing, China. A total of 10,942 athletes from 204 National Olympic Committees (NOCs) competed in 28 sports and 302 events (one event more than those scheduled for the 2004 Games). This was the first time that China had hosted the Summer Olympics, but the third time that the Games had been held in East Asia, following the 1964 Olympics in Tokyo, Japan, and the 1988 Olympics in Seoul, South Korea.
Document 12: London Underground: London Underground A deep - level Central line train at Lancaster Gate bound for Ealing Broadway A larger sub-surface Metropolitan line train at Farringdon bound for Aldgate Overview Locale Greater London, Buckinghamshire, Essex, Hertfordshire Transit type Rapid transit Number of lines 11 Number of stations 270 served (260 owned) Daily ridership 5 million Annual ridership 1.379 billion (2016 / 17) Website London Underground Operation Began operation 10 January 1863; 154 years ago (1863 - 01 - 10) Operator (s) London Underground Limited Reporting marks LT (National Rail) Technical System length 402 km (250 mi) Track gauge 1,435 mm (4 ft 8 ⁄ in) standard gauge Electrification 630 V DC fourth rail Average speed 33 km / h (21 mph)
Document 13: New Delhi: New Delhi is a major junction in the Indian railway network and is the headquarters of the Northern Railway. The five main railway stations are New Delhi railway station, Old Delhi, Nizamuddin Railway Station, Anand Vihar Railway Terminal and Sarai Rohilla. The Delhi Metro, a mass rapid transit system built and operated by Delhi Metro Rail Corporation (DMRC), serves many parts of Delhi and the neighbouring cities Faridabad, Gurgaon, Noida and Ghaziabad. As of August 2011, the metro consists of six operational lines with a total length of 189 km (117 mi) and 146 stations, and several other lines are under construction. It carries millions of passengers every day. In addition to the Delhi Metro, a suburban railway, the Delhi Suburban Railway exists.
Document 14: List of metro systems: This list of metro systems includes electrified rapid transit train systems worldwide. In some cases, metro systems are referred to as subways, U-Bahns or undergrounds. As of October 2014, 157 cities in 55 countries around the world host the approximately 160 metro systems that are listed here. The London Underground first opened as an ``underground railway ''in 1863 and its first electrified underground line opened in 1890, making it the world's first metro system. The metro system with the longest route length is the Shanghai Metro; the busiest one is the Beijing Subway; and the one with the most stations is the New York City Subway.
Document 15: Railway electrification system: On tracks that London Underground share with National Rail third-rail stock (the Bakerloo and District lines both have such sections), the centre rail is connected to the running rails, allowing both types of train to operate, at a compromise voltage of 660 V. Underground trains pass from one section to the other at speed; lineside electrical connections and resistances separate the two types of supply. These routes were originally solely electrified on the four-rail system by the LNWR before National Rail trains were rewired to their standard three-rail system to simplify rolling stock use.
ground_truth: City & South London Railway
sub_queries: where was the summer olympics held in 2012 Answer: London
what was the first underground line in London Answer: City & South London Railway