[3] The first section, 7.4 km (4.6 mi) in length and consisting of ten stations, connects the southwestern Kelenföld vasútállomás located in Buda, and the eastern Keleti pályaudvar in Pest, under the River Danube.

While three additional sections — the first, an eastern extension to Bosnyák tér, the second west to Virágpiac, and a third further east to Újpalota — have been planned, these remain unfunded by the Budapest city government and the European Union.

[6] In Hungary, the construction of the line has been widely criticised because its route was perceived as outdated, although the general city-structure and population density remained unchanged.

[9] The first decree was made in 1976 and the government intended to start construction in 1978, however the project was suspended in 1978, in favour of extending Line 3.

[19][20] The line was opened by Prime Minister Viktor Orbán on 28 March 2014, one week before the parliamentary elections that saw his party, Fidesz, reelected to a second supermajority.

[21] Construction of the line cost 1.5 billion Euros, or 1.5% of Hungary's annual GDP, of which 600 million came from European Union funds.

These funds, critics claim, would have been better invested in other large-scale transportation projects such as the connection of M2 to the Gödöllő HÉV or the construction of new tram lines.

[4] Some critics claim that without the extensions the current state of the line amounts to "several hundred-million forints thrown out the window.