Which Budapest metro line opened in 1896 as continental Europe's first underground railway?

The Millennium Underground Railway is the Budapest metro line that opened in 1896 as continental Europe's first underground railway.

Today designated Metro Line M1, or simply M1, the line was built beneath Andrássy Avenue to connect central Budapest with the City Park and the millennium celebrations commemorating the traditional 1,000th anniversary of the Magyar arrival. Construction began in 1894 and took less than two years.

The line opened on 2 May 1896 and was inaugurated by Emperor Franz Joseph shortly afterward. It used electric power from the beginning, making it the first electrified underground railway on the European mainland. The original route was about 3.7 kilometres long and had eleven stations, including underground and surface sections.

“Millennium Underground Railway,” “Metro Line 1,” and “M1” refer to the same historic line, so “Metro Line 1” is a common alternate answer rather than a different railway. The line remains in service and is known locally as the small underground, or kisföldalatti.