[5][6] The M1 line became an IEEE Milestone due to the radically new innovations in its era: "Among the railway's innovative elements were bidirectional tram cars; electric lighting in the subway stations and tram cars; and an overhead wire structure instead of a third-rail system for power.

Outside of the United Kingdom, the oldest fully underground urban railway in the world is the Tünel line in Istanbul, built in 1875.

The original line M1 ("Földalatti", from Hungarian föld "earth, ground", alatt "under"; so "the underground") ran for 5 km from Vörösmarty tér to Széchenyi fürdő.

[12] Line M1 was inaugurated on 2 May 1896, the year of the millennium (the thousandth anniversary of the arrival of the Magyars), by emperor Franz Joseph.

[13] It was named "Franz Joseph Underground Electric Railway Company" ("Ferenc József Földalatti Villamos Vasút Rt.").

The M4 (green) line commences at Keleti pályaudvar and travels south-west, crossing the river, to terminate at Kelenföld vasútállomás.

As part of the reconstruction, the Millennium Underground Museum in the old station at Deák Ferenc tér connected to the concourse.

[22] Line M2 runs east–west from Déli pályaudvar in Buda's Krisztinaváros, through the city center, to Örs vezér tere in eastern Pest.

The entire fleet of Metrovagonmash 81-717/81-714 and Ev/EvA carriages operating on the line were replaced with Alstom Metropolis metro cars by 2013.

[23] Planning of a direct connection of line M2 and the suburban railway lines with a shared new station at Örs vezér tere and the addition of a potential new underground station near Hungexpo Budapest Congress and Exhibition Center, offering another interchange point to mainline railways began in 2021.

[24] Line M3 runs in a north–south direction (more exactly, from north-northeast to southeast) on the Pest side of the river and connects several populous residential areas with the Inner City.

The upgrades included reconstructing the stations, rebuilding the track, safety equipment, ventilation and tunnel insulation.

[36] The internationally acclaimed 2003 Hungarian thriller Kontroll is set and was filmed in the metro system on the line M3.