Longer-term plans include the construction of a loop line connecting all existing lines. The plan also identified four subway lines crossing the Yangtze river. First line The initial section of Line 1 from Maigaoqiao to Xiaohang received final official approval in 1999. In May 2000, the Experimental Station resumed construction with the entire Line 1 project in full construction in December. A western extension of Line 1 from Xiaohang to a newly built stadium was fast tracked after Nanjing won hosting the National Games. Line 1's initial section and the western extension started trial operations on September 3, 2005, running from Maigaoqiao to Olympic Stadium with 16 stations and a total length of . The opening of the Nanjing Metro Line 1 gave Nanjing the sixth metro system in the Chinese mainland after Beijing, Tianjin, Shanghai, Guangzhou and Shenzhen. Current system Line 1 Line 1 runs mainly in a north–south direction. The line starts at Maigaoqiao in the north, heading southwards to CPU (China Pharmaceutical University). The construction of Line 1 began in the year 2000 and was inaugurated on September 3, 2005, with 16 stations and a length of . On May 28, 2010, Line 1's long south extension entered into operation. Thus, before the transfer of Line 1's Olympic Sport Center Branch to Line 10, Line 1 was long with 31 stations. The Olympic Sport Center branch line broke away from Line 1 and formed parts of Line 10, when the latter's construction finished and entered operation on July 1, 2014. Currently, Line 1 is long with 27 stations. Line 1's color is light blue. Line 2 Line 2 is long and has 30 stations. It runs mainly in an east–west direction, from Yuzui in the southwest to Jingtianlu in the northeast. It entered into operation on May 28, 2010. On 28 December 2021, the line was extended west with 4 new stations to Yuzui Station. Line 2's color is red. Line 3 Groundbreaking work for Line 3 started in January 2010. This line, with a north–south orientation, started operation on April 1, 2015 and is in length with 29 stations. Line 3's color is green. Line 4 Construction of east–west Line 4 begun in late 2012 and the first phase entered operation on January 18, 2017. The completed portion is in length. The line is known as A Zi (the Purple) as the branding for the line