It was opened in December 1990 as a metro/tram hybrid line named sneltram ('express tram') and served the suburb of Amstelveen.

Amstelveen still wished to continue with the plans for a metro as it would not require any demolition, but the topic was too controversial in Amsterdam.

The bureau released five reports by 1966 and laid out a final plan for a total of four metro lines in the city, which was presented to the public in the same year during a press conference by alderman Roel de Wit.

[1] The plan included a North–South line originating in Amsterdam-Noord, which would split into two branches in the suburb of Amstelveen and end in Schiphol-East.

[2] In May 1968, the municipal council of Amsterdam approved the plans based on the advice of the bureau and reserved the money required for the first phase of the project, the two East–West lines.

[3] The metro was a controversial topic as the houses above the underground sections of lines in the city centre of Amsterdam had to be demolished to make construction possible.

[12] In November, the Ministry of Transport and Water Management announced that it would only support the express tram if it was extended to Amsterdam Centraal station instead of ending at Zuid, resulting in less transfers.

[13] In February 1985, the two municipalities agreed on building the express tram; funding from the Dutch Government was secured shortly after.

[14] Centraal Nederland still started a campaign to construct a dedicated bus lane instead, which they claimed would be cheaper than the tram.

[17] Construction on the line started in August 1987 and was met with protests by houseboat residents at Duivendrechtsevaart, who had to move out.

After Zuid, the trains would extend their pantographs and pick up current from 600 V DC overhead lines to serve Amstelveen as a tram.

[25] The second part of the route also shared tracks with tram line 5[8] and was constructed on the median strip of a road.

[30] The capacity of the remaining trains was not enough and some people were left behind on the platform during rush hours due to the vehicles being full.

[31] The route of the line was shorted from Centraal to Zuid in February 1991 as 11 of the units were not running due to technical problems caused by the winter.

[34] Following the resumption of service, regular busses still ran just behind the express trams in case a problem occurred, as the rolling stock was sensitive to problems when switching operation modes, electronics froze during cold days and some communication cables were chewed by rabbits.

[39] An express tram caught fire at the Weesperplein station on 12 July 1999 due to a blocked disc brake.

Although the tram was carrying no passengers at the time, the smoke coming from the fire caused all levels of the station to be evacuated.

[27] The same study found that it was "essential" to upgrade the Amstelveen Line to modernise the metro system of Amsterdam.

Wiebes had imposed a cost–benefit analysis which found that the upgrade to a full metro would be too expensive and unaffordable, while the office kept pushing for the conversion.

[50] On 9 February 2012, the province of North Holland and the municipality of Amsterdam agreed to relocate approximately 1 kilometre (0.62 mi) of the A10 motorway underground in a tunnel.

[51] On the same day, the municipalities of Amsterdam and Amstelveen announced their support for the conversion of the line into a high-quality tram costing around 300 million euros.

[53] The transport authority made the decision to continue with this option in December 2015[54] after the two municipalities voted in favour of it in November.

Metro 51 kept its original route from Amsterdam Centraal to Zuid, and instead of branching off to Amstelveen, it continues west-bound and ends at Isolatorweg.

[63] On 30 March 2020, the GVB temporarily suspended the metro 51 as a result of the COVID-19 pandemic in the Netherlands impacting the number of passengers;[64] service was resumed on 29 April.

[74] GVB put the blame of the capacity and technical issues that occurred after the opening of the line on the constructor and considered asking for financial compensation.

[77] Following the conversion of the express tram, the S1/S2 trains went through a technical upgrade for the final time and were used on regular metro lines.

[82] BN had also made an offer to deliver new units, but was not selected due to regular problems with the previous express trams.

23 of these trains were ordered in 2010 to replace the oldest type of metro in Amsterdam, the M1/M2/M3 series, as well as the brand new North–South Line.

[88] The trains include a higher ceiling and door height than the standard Metropolis units running in other countries.

[70] M7 are 59.6 metres (196 ft) long trains based on the Inneo series of Construcciones y Auxiliar de Ferrocarriles and consist of 3 cars.