Andy Garcia · freelance dev
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Engineering

Paris Line 18's First Leg Is Due in December. Lines 16 and 17 Must Trail Line 15 by 9 to 12 Months

October 20, 2026 8 min read

On Thursday, September 24, 2026, the Société des grands projets (SGP), the public body building the Grand Paris Express, told its supervisory board that the first section of metro Line 18 is scheduled to open in December 2026. Four stations, Massy-Palaiseau, Polytechnique, Université Paris-Saclay and Christ de Saclay, between Massy and the Saclay plateau south-west of Paris, with 6.7 kilometers of viaduct. SGP says it will serve the plateau's 70,000 students, teacher-researchers and employees. It is the first phase of a 35-kilometer, 10-station line due to link Orly airport to Versailles by 2030, according to Île-de-France Mobilités (IDFM), the regional transport authority.

SGP gave no day. In June, it wrote that service could start "from November 30", once the required safety authorizations were obtained. In September, it said December. On September 29, Valérie Pécresse, who chairs IDFM, said mid-December, and tied the opening to a budget dispute with the State. It would be the first new metro line in the Paris region since Line 14 in 1998, as the line's director at SGP, Vianney Elzière, put it.

The same board meeting moved two other lines. Lines 16 and 17 will first open in autumn 2028, about a year after Line 15 South, which SGP had already pushed in June from April 2027 to autumn 2027. The reason is one sentence of the communiqué: Lines 15, 16 and 17 are driven by the same automation system.

I don't build metros. I ship software, as CTO of a LegalTech built on Claude and as an indie developer. I read SGP's two board communiqués, its March newsletter on Line 18 and Alstom's 2021 contract announcement with one question: once the concrete is poured, where does the calendar go?

What "ready" meant on Line 18

Civil works were not the bottleneck. On July 17, SGP reported that works on the section's three elevated stations were finished and that safety commissions had cleared them to receive the public; Massy-Palaiseau, the fourth, was due to mark the end of its works on September 12. By September 24, SGP said all four stations had received a positive opinion from the safety commissions over the summer.

Testing had started a year earlier. SGP's March newsletter lays out the sequence. Static tests in July 2025. From December 18, 2025, one train with a driver aboard, never above 30 km/h: the "integration" phase, meant to check that the pieces of equipment work together. At that pace: gauge tests, one train towing another, braking, the third rail that powers the train, the beacons a driverless train reads along the viaduct. Then driverless runs, several trains at once, up to 110 km/h. By March: 1,000 hours of tests.

Then come gates that are not engineering in the narrow sense. Safety commissions, which allow each station to receive the public. The "transfert en gestion technique", when SGP hands the keys to IDFM and its operator, Keolis. Then the "marche à blanc": two months of trial running without passengers, by Keolis and RATP Infrastructures, the maintainer.

The March newsletter put the safety commissions in spring, the handover and trial running in summer, and the opening in October 2026. Elzière told the trade magazine Ville, Rail & Transports that the common target for the handover had been July 31, and that in June "not all the conditions were met". The June communiqué rescheduled the handover for September 30, the September one for October 13. July 31 to October 13 is 74 days. The opening moved from October to December.

SGP points out that the 2026 target itself has not changed since 2018. The year held. The last steps took the slack.

One of those steps cannot be compressed. The safety file of the transport system, under review by the State's services, was declared complete on September 3. It will be completed with evidence from the trial running, so that the last safety commission can be held. Working harder does not make two months of trial data arrive sooner.

Line 15 South: installed is not integrated

The June communiqué is candid about Line 15 South, 16 stations on a 33-kilometer arc. All 255 escalators are installed. Train speed tests took place in March, with qualification tests meant to validate 80% of the line's functional systems. And still, two paces were too slow: the deployment of train automation equipment in the tunnels, which conditions the start of system testing, and the integration of the 16 stations "and all the equipment that composes them", a complex process that demands heavy mobilization from the contractors on the critical path.

AFP gave the scale. Each station has about 700 pieces of equipment, from cameras and lifts to automatic-driving electronics and signaling, connected to the central control room. In 2024, 3% of Line 15 South's equipment was connected, Jean-François Monteils, head of SGP's management board, said in June; by then it was "almost 100%". Sixteen stations times 700: roughly 11,200 devices to connect and prove, on top of trains and track. AFP also reported SGP's account of the delay: three months lost when the integrating design office was changed in 2024, and three more of "inertia" among the 835 companies on the project.

SGP's answer was not more concrete. It revised the contract architecture so contractors could be prioritized, and reconfigured its test strategy to test the transport system and the station systems in parallel.

As an analogy, and only that: this is the distance between "every service is deployed" and "every end-to-end test passes". The first counts parts. The second counts joins.

One system, three calendars

In September, SGP assessed what the Line 15 South delay means for the two lines sharing its automation system. The work "confirmed a necessary interval of 9 to 12 months" between the opening of Line 15 South and that of Lines 16 and 17. The transport system of all three is deployed by the ST4GP consortium, led by Siemens Mobility France.

SGP also redrew the phases. The first, in autumn 2028, runs from Saint-Denis Pleyel to Parc du Blanc-Mesnil on Line 16 and to Le Bourget Aéroport on Line 17. Until then, the whole of Line 16 had been due at the end of 2027 and that first stretch of Line 17 in autumn 2027, ICI Paris Île-de-France reported. The aim is to secure the section the two lines share, with the Aulnay operations center and the fork, "a significant technical challenge in itself", SGP writes. The second phase follows in autumn 2029; the last stretch of Line 17, to Le Mesnil-Amelot, stays set for the end of 2030.

Line 18 sits outside that chain. In October 2021, Alstom announced a single contract for its trains, driverless train control, data transmission and centralized supervision, plus the role of general integrator: verifying each subsystem's tests and organizing whole-system tests. The firm phase was worth about 230 million euros.

The documents do not show that this is why Line 18 is due first; SGP never says so. Line 18's first section has four stations, not 16, and SGP's newsletter calls its 6.7 kilometers of viaduct the section's "specificity". Contract structure, size and geometry are all mixed in this comparison. And Line 18 slipped too.

The handover is a project of its own

The part I reread is Elzière's description of the organization. The builder, SGP, is not the transport authority. The authority, IDFM, designated an operator, Keolis, which is not the maintainer, RATP Infrastructures. In Lyon, Rennes, Lille or Marseille, he said, there are two actors: an authority and an operator-maintainer. "Here, we have set up a ménage à quatre."

Nicolas Samsoen, chair of SGP's supervisory board, called the September slip "an adjustment in number of days to let the operator finish training the staff". IDFM said it would make sure everything works correctly "before accepting the transfer to the operator".

SGP wrote the lesson down. Citing the experience of Line 18 on "coordination between the project's stakeholders", the board set up a working group for Line 15 South nearly a year before its handover: SGP, its systems contractor, IDFM, RATP Infrastructures and ORA, the consortium designated to operate it.

In its interview, published on October 12 and updated on October 16, Ville, Rail & Transports reported that the handover had taken place and Keolis's trial running had begun. I found no SGP or IDFM statement giving the exact day. Money is open too: on September 29, Pécresse said that losing 90 million euros of 2027 revenue in the State's draft budget could force IDFM to delay new lines, starting with Line 18. On October 2, once the bill was out, IDFM put the loss at about 90.2 million euros, from a shift of tourist-tax revenue to SGP in its article 42. As of October 20, I found no announcement settling it.

What I would copy

  1. Put integration on the plan as its own phase, with an owner and a date, and measure it in joins, not parts.
  2. Treat a shared platform as a shared critical path. If three products wait on one system, its first production deployment is everyone's deadline. Prove the hardest shared piece first, on a reduced scope.
  3. Know who signs the last gate and which evidence they need. If it is trial-run data, the trial's length is a floor, not an estimate.
  4. Start the handover early. The people who will run the system need training time that no amount of engineering removes.

If Line 18 opens in December, it will not prove that building a metro is easy. It will show where the last months of six years of works went: into the joins, the paperwork, and the people who will run it.

Sources

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