FRANCE: The first MI20 double-deck train for Paris’ RER Line B has been unveiled at Alstom’s Crespin plant, taking the fleet renewal programme for one of Europe’s busiest commuter lines into its test phase ahead of the much-delayed rollout.
The Paris region’s public transport authority Île-de-France Mobilités has marked the first outing of an MI20 trainset for the Paris RER B line. The unit was formally presented at Alstom’s manufacturing site in Hauts-de-France, where the trains are being built, offering a visible milestone for a fleet renewal programme that is vital to the French capital, but has been surrounded by delays and acrimony.
The double-deck unit is part of a 146-train fleet intended to replace ageing rolling stock on the north-south Paris regional line, which serves central Paris, its northern and southern suburbs, Charles de Gaulle Airport and Orly Airport. One of Europe’s heaviest-used urban rail lines, RER B already carries close to one million passengers a day across the Paris region, with growing demand having long strained the existing rolling stock.
Île-de-France Mobilités is therefore putting around half of a wider €5bn investment programme for RER B into the MI20 fleet — the rest is set for a new command centre shared with RER D and the deployment of NExTEO train automation technology — with the new trains set to increase onboard capacity by 35%.
The trains are finally starting to arrive. © Île-de-France Mobilités
Long-awaited passenger upgrade
For manufacturers Alstom and CAF, as well as Île-de-France Mobilités, the first outing of the new unit provides a long-awaited sign of progress after repeated problems with the MI20 programme. Low-speed testing has now started, with higher-speed dynamic testing due to begin this summer. However, the trains are still not expected to enter service until the end of 2028, almost three years later than originally planned. And that itself could still be an optimistic timeline.
MI20 fleet at a glance
- Fleet size: 146 new double-deck regional trainsets
- Line: Paris RER B
- Manufacturers: Alstom and CAF
- Funder: Île-de-France Mobilités
- Operators: RATP and Transilien SNCF Voyageurs
- Investment: around €2.5bn, within a wider €5bn RER B upgrade
- Replaces: ageing MI79 and MI84 rolling stock
- Capacity: up to 35% more onboard capacity, 22% more seating
- Accessibility: 100% accessible, with level-access areas, wider corridors and more doors
- Comfort: air-conditioning, heated floors, quieter operation
- Passenger systems: USB-C sockets and real-time information screens
- Security: onboard video surveillance with live transmission
- Testing: low-speed tests under way; higher-speed dynamic tests due this summer
- Service target: test running on RER B tracks in 2027, passenger service for end of 2028
Funded by Île-de-France Mobilités and set to be operated by RATP and Transilien SNCF Voyageurs, the MI20 promises the kind of passenger upgrade RER B has long needed, with more space to board and move through the train, level-access areas, better temperature control and USB charging on board. But the first unit’s arrival at Crespin also makes clear how long that upgrade has taken to reach even the test phase.
Delays turn political
By autumn 2025, the MI20 delays had become a major political issue in Paris. The trains had originally been due to begin entering service in 2025, before the target slipped to 2027 amid production difficulties and co-ordination issues between Alstom and CAF. An expert review commissioned by the Paris transport operators then warned that, without major changes, the first units might not be ready before September 2029.
President of the regional council of Île-de-France Valérie Pécresse is happier than last year. © Île-de-France Mobilités
That warning drew a public rebuke from Île-de-France Mobilités, with President of the Regional Council of Île-de-France Valérie Pécresse calling the latest slippage ‘unacceptable’, reminding Alstom that IDFM was its ‘largest customer’ and that contractual deadlines therefore had to be respected. Alstom was keen to show that the delay was not solely of its making, pointing to CAF’s role in delivering the vehicles needed for first assembly, but that did little to ease the pressure around the programme.
A bespoke train for a difficult line
Part of the difficulty lies in the RER B itself. The line is jointly operated by RATP and SNCF Voyageurs, with trains moving between different infrastructure environments, signalling conditions and operating rules, as well as two electrification systems: 1.5 kV DC and 25 kV AC.
The route also carries physical constraints inherited from the former Sceaux line, the historic railway that now forms part of the southern section of RER B, including restricted tunnel and platform clearances. That means the MI20 could not simply be lifted from the RER NG programme, forcing the manufacturers to develop a narrower and lower train for the route.
Even the consortium structure carried its own complications. Alstom inherited the MI20 partnership after acquiring Bombardier Transportation, which had been part of the original 2020 bid with CAF; although Alstom later tried to challenge the arrangement, the award was confirmed and the French manufacturer remained tied to the split-build structure.
A more roomy layout. © Île-de-France Mobilités
Under that structure, CAF is responsible for the motor cars, while Alstom builds the intermediate cars and leads integration, assembly, traction equipment and testing, leaving the programme dependent on tight co-ordination between the two manufacturers before the trains can move through approval.
Those interfaces are key because the traction, braking and control systems have to work together as a single train before the fleet can be certified. On a route with two operating environments and strict clearance limits, even relatively small design changes can create major additional validation work. Alstom has itself pointed to that dependency, telling RailTech last autumn that the first MI20 would be assembled in the second quarter of 2026, ‘provided that the final vehicles supplied by CAF are delivered on time.’
Getting back on track
Work at the factory continues. © Île-de-France Mobilités
Alongside its public rebuke of Alstom, IDFM put forward an emergency action plan designed to bring the MI20 programme back under control, with binding milestones, revised governance and tighter oversight of production, testing and approval.
Alstom later told RailTech it was working with the RATP-SNCF group on a shared schedule intended to minimise risk and preserve entry into service before the end of 2028, while saying production had already begun on 10 trains: five test trains for the consortium, three test trains for the customer and the first two commercial units.
With the first train now presented at Crespin, that part of the recovery timetable appears to have held. However, the programme still has to move from low-speed testing into dynamic tests, safety validation, customer acceptance and passenger service before the new fleet can begin carrying passengers on RER B.
And then there is the issue of the MI20 being technically ‘married’ to NExTEO, the €2.5bn digital signalling and automation project designed to increase frequency on the line. Because the ageing MI79 and MI84 fleets currently running on the line are incompatible with this new technology, any further delays in the rolling stock rollout effectively ‘bricks’ the wider modernisation programme.
’10 years of fighting’
10 years of fighting, but Pécresse is not at the end yet. © Île-de-France Mobilités
Pécresse’s own response to the factory presentation went some way to capturing that mix of relief and unfinished business. After her criticism last autumn, the Île-de-France president said this week that she could ‘not hide my emotion after 10 years of fighting against bureaucracy, standards, hazards and schedule delays’ to see the first complete MI20 leave the factory.
Still, testing, she said, now has to continue so that the trains can run on RER B tracks in Île-de-France at the end of 2027, before the latest planned entry into passenger service the following year. There has been progress, but the programme is still a long way from carrying passengers.