Metro Line M1 (Budapest Metro)
Line 1 (Officially: Millennium Underground Railway, Metro 1 or M1) is the oldest line of the Budapest Metro, built from 1894 to 1896. It is known locally as "the small underground" ("a kisföldalatti"), while the M2, M3 and M4 are called "metró".
Source: Wikipedia — Metro Line M1 (Budapest Metro) (CC BY-SA 4.0)