The Underground Railway Museum (Hungarian: Földalatti Vasúti Múzeum), also known as the Millennium Underground Museum, is a museum located under Deák Ferenc square in the centre of the Hungarian capital city of Budapest. It is accessible from the pedestrian subway system that links the square to Deák Ferenc tér metro station and is housed in a tunnel that once carried the tracks of line M1 of the Budapest Metro.

Line 1 is the oldest of the metro lines in Budapest, having been in constant operation since 1896. The initial section ran between Gizella tér station (now Vörösmarty tér station) and Aréna út station (now Hősök tere station) and served an intermediate station under Deák Ferenc square. This original station was constructed with two side platforms, at a depth of just 2.7 metres (8 ft 10 in). The approach to the station included a sharp curve disctated by the original layout of the square.^[1][2]

In the 1950s and in preparation for the planned line M2 of the metro, the route of the M1 line under the square was diverted in order to ease the sharp curve. This diversion left an 80 metres (260 ft) long section of the original tunnel empty and walled off. In the end, line M2 did not open until the 1970s, but in the 1960s further preparatory work led to the creation of a large-scale pedestrian underpass system under the square, which called in question the fate of the section of abandoned tunnel. At first it was planned to fill it in, but then it was suggested that a metro exhibition be organized in it.^[2][3]

As the eventual result of this suggestion the new museum was created in the rebuilt tunnel. The museum opened on 26 October 1975. It was renovated in 1996, the 100th anniversary of the opening of the railway.^[2][3]