Grand Paris Express tests line 18 on viaduct, service from end of 2026

On 18 December 2025 the first train set off on test runs over the 6.7 km viaduct of Grand Paris Express line 18, the first time on an elevated section of the network; the Société des grands projets, together with Alstom and Egis, is checking among other things rescue by a second train after a breakdown, braking and acceleration up to 30 km/h and the reading of signals. Automatic-mode running follows in early 2026, then a dress rehearsal by the operator Keolis; commissioning between Massy–Palaiseau and Christ de Saclay is planned for the end of 2026. The date is a placeholder: the article gives only the end of 2026. (LOK Report, 31 Dec 2025)