Sofia

The red line and the first to be built on the Sofia Metro, the only metro system in Bulgaria. Its first 6.5 km with five stations, from Slivnitsa through the Lyulin housing estates to Konstantin Velichkov, opened on 28 January 1998; Opalchenska followed in 1999 and Serdika, under St Nedelya Square, in 2000, taking the system to 8.1 km. Sections begun in 2005 and 2006 carried the line east under the city centre to Mladost 1 in 2009, and a 2.7 km extension reached Business Park Sofia in May 2015, bringing the line to 17.6 km and 16 stations. Between Slivnitsa and Mladost 1 it shares its tunnels with line M4, which branches away to Sofia Airport; a westward extension beyond Lyulin is in planning but has not been contracted.

The blue line, Sofia's second metro diameter, running 11.7 km with 13 stations from the Obelya estate in the north-west through Nadezhda, the Central Railway Station and the National Palace of Culture to Lozenets and Hladilnika in the south. It was built as the first two EU-funded stages of the metro: a 6.4 km central section with seven stations, partly bored by TBM, and a 4.16 km northern section with four stations, both opened together on 31 August 2012. A 1.3 km extension to Vitosha followed on 20 July 2016. Trains run through at Obelya onto line M4 towards Sofia Airport; the two lines will be separated once the infill station Pancho Vladigerov opens.

The green line, Sofia's third metro diameter, linking Ovcha Kupel and Gorna Banya in the south-west with the city centre and the Hadzhi Dimitar and Levski estates in the north-east. Unlike the older lines it uses 1,500 V overhead catenary and driverless Siemens trains behind platform screen doors. The 7.8 km central stage with eight stations opened on 26 August 2020 and a 3.8 km western stage with four stations followed on 24 April 2021, at a combined cost of about €500 million with EU grants covering roughly four fifths. A 3 km, three-station extension under Gen. Vladimir Vazov Blvd to the Levski estate, built from March 2022 with Recovery and Resilience Facility money, is complete and due to carry passengers from 14 August 2026. A six-station branch through Slatina, numbered M6, splits east of Orlov Most and is still under construction.

The yellow line, 23.0 km with 20 stations, from the Obelya estate across the city to Terminal 2 of Vasil Levski Sofia Airport. For most of its length it has no tunnels of its own: between Slivnitsa and Mladost 1 it uses the line M1 tunnels built between 1998 and 2009. It is a branch in its own right only at the ends, in the 1.8 km Obelya spur opened in April 2003 and the eastern extensions to Tsarigradsko shose (2012) and the airport (2 April 2015). Trains through-run at Obelya onto line M2; until 26 August 2020 the airport service was signed as part of M2, and it was renumbered M4 in preparation for the infill station Pancho Vladigerov, which will physically separate the two lines and let them keep independent timetables. Since 19 July 2025 M4 has terminated at Slivnitsa while that station is built.