M51, also known as the Ring Line, is an Amsterdam Metro line running from Amsterdam Centraal station to Isolatorweg metro station, almost creating a full circle through Amsterdam. It was opened in December 1990 as a metro/tram hybrid line named sneltram and served the suburb of Amstelveen. Using special rolling stock, the Amstelveen Line ran as a metro on 750 V DC third rail from Amsterdam Centraal to the Amsterdam Zuid station, where it would switch operation modes and utilize 600 V DC overhead lines and end at Poortwachter. An extension to Westwijk was completed in 2004. In Amstelveen, the line partially ran on the same track as tram line 5 and had many level crossings.

The Amsterdam Metro is a rapid transit system serving Amsterdam, Netherlands, and extending to the surrounding municipalities of Diemen and Ouder-Amstel. Until 2019, it also served the municipality of Amstelveen, but this route was closed and converted into a tram line. The network is owned by the City of Amsterdam and operated by municipal public transport company Gemeente Vervoerbedrijf (GVB), which also operates trams, free ferries and local buses.