The project, which was granted public utility status last year, involves constructing an 8km tunnel from St Lazare to La Défense and upgrading the existing line to Poissy and Mantes-la-Jolie. The extension will cut journey times between western suburbs and La Défense, an important commercial centre on the west side of Paris, by up to 17 minutes. It will also provide existing Line E passengers with direct access to La Défense, and will relieve congestion by between 10 and 15% on the Auber – La Défense section of RER Line A, which Line E will parallel, and part of RER Line C.

The extension will interchange with RER lines A and C, metro Line 1, light rail Line T2, and the future Orange, Red and Green lines of the orbital Grand Paris Express network. With the opening of the extension, Line E is expected to carry 89,000 passenger per hour in the morning peak and 620,000 passengers a day by 2022.