Line 18 is the result of the Grand Paris public transport network project, presented by the President of the Republic Nicolas Sarkozy in 2009.

This provided for a large gauge green line connecting Orly Airport to Roissy via Saclay and La Défense.

The green line is then included in the Grand Paris Express project with a terminus this time fixed at Nanterre, with entry into service then envisaged between 2025 and 2030.

[7][8] On 15 May 2020, the Société du Grand Paris awarded Vinci Construction the first civil engineering contract for the line 18, for an amount of 800 million euros: it concerns the section from Orly to the Saclay plateau.

[17] The line starts underground at Orly Airport to the south-east at the limit of the departments of Essonne and Val-de-Marne, under the car parks between the west and south terminals.

The line slants again towards the west and enters the town of Palaiseau before passing under the D36 then coming to stand parallel to the latter at the level of the district "Camille-Claudel."

Finally, the line passes under N12 and heads north-east before running alongside the railway tracks and arriving at the terminus of Versailles – Chantiers where it connects to RER C and Transiliens U and N.[20]

An operations center playing the role of maintenance and storage site for trains is planned at Palaiseau, along the departmental route 36, near the École Polytechnique campus.

[22][23] In addition, the inhabitants of Wissous, a town located between Antony and Orly Airport, crossed by the line but not served by it, in fact feel forgotten by the project.

[25] In addition, the analysis of the socio-economic characteristics of the territories crossed shows that we have in the Orly pole " a high proportion of workers "on the spot", that is to say holding a job.

The FNAUT Île-de-France transport users association considers line 18 as a prestigious metro disconnected from real needs travel.

[28] According to Pascal Auzannet, former senior official in charge of reflections on the Grand Paris metro project within RATP in the 2010s,[29] it would be wiser to extend the 7.3 kilometers from the Orlyval to the Saclay plateau.

Indeed, with a transport capacity at peak hour greater than 7500 passengers, this possibility is compatible with the traffic forecasts presented in 2016 during a public inquiry.

[31] In July 2018, the Council of State rejected the requests of several communities and associations for the defense of the environment and validated the declaration of public utility.

[32] In July 2020 during the last public inquiry into the project, FNAUT Île-de-France Transport Users Association pointed out in a message posted on the microblogging network Twitter[33] that the socio-economic profitability of this line has been revised upwards in an unexplained manner.

[34] The report of the Public Inquiry Commission of 2016 recommended that a land reserve be set up for the possible realization of this project after the opening of the line.

This measure is taken after requests from local elected officials of Val-de-Marne and Essonne to connect Villeneuve-Saint-Georges, located on RER D, to the Orly Airport area to facilitate its access.

[37] The public inquiry commission recommends during its publication of 13 July 2016 that the Aéroport d'Orly station be built in such a way as to make possible an extension to RER D.[35] Line 18 will serve several notable points of Île-de-France: The proposed rolling stock for line 18 is the MRV (Matériel Roulant Voyageurs), a new automated design with a width of 2.45 metres (8 ft 0 in), using conventional steel wheel on steel rail technology and third rail electrification.