YDS READING PASSAGES / TEST 4 58. Soru

-20. THE MADRID METRO

The Madrid metro is the largest metro system serving Madrid, the capital of Spain. It is one of the largest metro systems in the world, which is especially remarkable considering Madrid's less than four million population. It is also one of the fastest growing in the world, rivalled only by Seoul; the latest round of expansions, completed in the spring of 2003, have increased its length to 223 kilometres. The metro opened in 1919 under the direction of the Compania de Metro Alfonso XIII. Metro stations served as air raid shelters during the Spanish Civil War. Starting in the 1970s, it was sequentially greatly expanded to cope with the influx of population and urban sprawl from Madrid's economic ascendancy. A huge project in the late 1990s and early 2000s installed approximately 50 kilometres of new metro tunnels, including a direct connection between downtown Madrid and Barajas International Airport, and service to outlying areas, including a huge 40-kilometre circuit called Metrosur serving Madrid's southern suburbs. Metrosur, the largest civil engineering project in Europe, opened on April 11, 2003. It includes 40.7 kilometres of tunnel and 28 new stations, including an interchange station and an additional station on Line 10, which connects it to downtown. Construction began in June 2000, the whole circuit was completed in less than three years. It connects the towns of Getafe, Möstoles, Alcorcon, Fuenlabrada and Leganes. Madrid also has an extensive commuter train network operated by Renfe, the national rail line, which is integrated with the metro network. Several commuter train transfer stations were included in Metrosur.

From the author's statement, we can infer that the size of Madrid's metro system is ------ .