Red Line
The first 6.5 kilometres (4.0 mi) long section of line 1 consisting of five stations linking Slivnitsa Boulevard through Lyulin and K. Velichkov Boulevard was inaugurated on 28 January 1998. Opalchenska station entered into service on 17 September 1999 and Serdika station situated on St Nedelya Square followed on 31 October 2000, extending the total system length to 8.1 kilometres (5.0 mi). The operational section of the line was further extended with a 1.8 kilometres (1.1 mi) long section, reaching Obelya housing estate in April 2003.
The extension of Line 1 continued in 2005 with the start of the construction of 4.8 kilometres (3.0 mi) of tunnels and three stations linking St Nedelya Square and the Interped World Trade Center in Izgrev (station Frédéric Joliot-Curie). 2006 saw the start of the construction of another section of the same line (consisting of 3.2 kilometres (2.0 mi) of tunnels and three stations) linking Izgrev and Mladost I housing estate. The completion of the first three stations was projected for the autumn of 2007, however as a result of various delays it was the second section from the first line (Vasil Levski stadium - Mladost 1) that first entered into service on May 8, 2009, operating for a brief period of time separately from the north-west portion of the line. The remaining section between Serdika and Vasil Levski stadium station finally entered into service on September 7, 2009 establishing an interrupted link between Obelya and Mladost 1 stations.
Line 1 branches in two: Northern branch from Mladost I to Sofia Airport (7.2 kilometres (4.5 mi), six stations), and Southern branch from Mladost I to Business Park Sofia (four stations). The construction of the first two stations of the Northern branch began on 15 February 2009 and was completed on 25 April 2012. The Northern branch extension to Sofia Airport is under way and it is planned to be completed and enter into service by 2015. The construction of the Southern branch to Mladost IV and the Sofia Ring road began on 25.04.2013.The stretch is expected to be ready in two years. It is 2,7km long, costs EUR 120,000,000 and is going to service the majority of the second most densely populated area in Sofia.
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