SNCF Réseau is committed to installation of the digital NExTEO train control system to increase train throughput on the busiest sections of the heavily used Paris RER network. Jérémie Anne reports.

Paris is distinguished by having one of the world’s busiest rail networks. Although it represents just 10% of the French network in terms of length, the Paris area handles 40% of all trains in France and carries 70% of all passengers. Several routes in and through the French capital are already heavily congested, so much so that infrastructure manager SNCF Réseau has been seeking ways to increase throughput.

The option of adding extra tracks was ruled out because of the high cost and potentially long timescale to build them in a busy and densely populated area. SNCF Réseau therefore opted for the NExTEO (Nouvelle Exploitation Transilien Est-Ouest) train control system.

As first described in April 2023, NExTEO is based on communications-based train control, as developed for numerous metro networks around the world. In contrast to conventional railways which operate with mixed traffic in an open environment, metro services generally run in a closed environment. Applying CBTC on a conventional network therefore represents a significant challenge, says SNCF’s project director for NExTEO, Jérôme Lefebvre.

NExTEO is based on a moving block concept with elimination of lineside signals and cab signalling. There are three distinct elements or layers, explains Lefebvre. First, automatic train protection, which guarantees operational safety by controlling the train speed and communicating with the signalling equipment. Second, automatic train operation is used to drive the train, governing acceleration and braking and assisting or overseeing the actions of the train driver. Third comes automatic train supervision, which manages traffic in real time by monitoring operation of the trains.

CBTC basis

In terms of its design and conception, NExTEO conforms to Grade of Automation 2. As with all types of CBTC, it features high performance continuous communication between onboard and wayside equipment using radio links. The onboard apparatus comprises a NExTEO specific transmission module which interfaces with a European Vital Computer and an odometer fitted to one of the train’s wheels. Using radio antennae and equipment that reads Eurobalises, the STM and the odometer establish a dialogue with the wayside equipment, communicating the necessary information about the train.

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The wayside equipment consists of Eurobalises which interact via a radio network with the zone controller in the signalling centre. The controller calculates the length of track over which the train can safely run and transmits this information to the onboard equipment, which adapts the speed of the train accordingly.

Lefebvre points out that the transition between sections of track equipped with NExTEO and those without it is automatic. In this way NExTEO allows assisted driving or manual control, with the driver responding to the speed advice generated by NExTEO.

NExTEO was designed at a time when ERTMS and its deployment were considered to be a priority for the European Union. It has nevertheless been classified as a Class B system. Lefebvre does not question the existence of the two systems, noting that ERTMS is the right choice for other operating situations.

The density of traffic in the Île-de-France area is such that NExTEO was considered to be the only viable option. Using ERTMS in this exceptionally busy area would represent ‘a bet on the future’, especially as performance in such a heavily trafficked environment is not yet guaranteed. CBTC is considered to be a ‘mature’ technology for the expected traffic flows, Lefebre explains.

NExTEO will be fully compliant with the Technical Specifications for Interoperability. Under the new TEN-T regulation, Class B systems will have to be dismantled by 2040, but NExTEO should not be affected as it is designed specifically for application with urban passenger transport.

First application

In February 2016 SNCF awarded a contract for the train control system for the central section of RER Line E between Pantin and Nanterre-La-Folie to a consortium of Siemens Mobility and Atos. This included the extension from Haussmann-Saint-Lazare to Nanterre-La-Folie, which opened in May 2024. By the target date of November 2024, it should be possible to run 16 trains/h in each direction through the tunnel without recourse to NExTEO, as the conventional colourlight lineside signalling with automatic block and KVB train protection will suffice. Trains will run at a maximum speed of 120 km/h, except between Rosa Parks and Haussmann where speed will be limited to 80 km/h.

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In contrast, when the next stage of the Line E project is completed with trains running from Nanterre to Mantes-La-Jolie, provisionally in late 2026, the use of NExTEO will be essential. This is because there will be a further six trains/h to and from Mantes, meaning that the central section of the line must accommodate 22 trains/h in each direction. The minimum interval between trains over this section will be 108 sec.

Rolling stock will consist of a fleet of 125 RER NG trainsets supplied by Alstom; these will be the only trains fitted with NExTEO.

Trials and validation

A test installation has been built over a 1∙8 km section at Gagny. This replicates all the possible NExTEO situations, and the first trips entirely controlled by NExTEO were made during 2023.

Using two dedicated RER NG trainsets, around 1 000 km have so far been operated with NExTEO, says Lefebvre, both as single units and with the two trains running in multiple. Assuming all goes well, the French railway safety authority EPSF will approve the system for application in commercial service.

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On certain sections, for example from Pantin to Val-de-Fontenay and from Pantin to Le Raincy-Villemomble-Montfermeil, the existing signalling will be retained as traffic density is lower. Nonetheless, an ATS+ system, also supplied by Siemens-Atos, will be installed. This will control all traffic in the area, including Line E RER services, TGVs, Line P suburban services, Grand Est TER trains and freight. Apart from improving traffic management, the intention is for ATS+ to ensure that RER Line E trains are presented on time in the correct sequence before entering the central section equipped with NExTEO.

Lines B and D next

Now known formally as Nouvelle Exploitation des Transiliens, NExTEO is also to be fitted to two other intensively used lines in the Paris area. These are RER Line B, whose control is shared between RATP at the southern end and by SNCF in the north, and RER Line D, which is operated solely by SNCF. Sharing tracks between Châtelet and Gare du Nord, the two lines together carry 1∙6 million passengers a day. Managed by RATP, the two-track Châtelet – Gare du Nord tunnel handles 20 Line B and 12 Line D trains/h in each direction during peak periods.

With 32 trains/h each way, this 2∙6 km long tunnel is the busiest section of railway in Europe. Traffic on both RER lines is easily disrupted, not least because of the shared use of the central tunnel section. Various studies have looked at how best to deal with the situation, and in every case NExTEO was considered the best answer.

Around 100 route-km are to be equipped. Line B will be fitted from Aulnay-sous-Bois to the southern termini at St-Rémy-les-Chevreuse and Robinson. Line D will be fitted from Stade de France St-Denis to Villeneuve-St-Georges. Installation will be carried out section by section, with the target completion date ‘during the 2030s’, according to Lefebvre.

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All rolling stock using the two lines will be fitted for NExTEO: 132 RER NG trainsets for Line D supplied by Alstom and 146 MI20 Line B sets built by an Alstom-CAF consortium. The headway between trains in the tunnel section will be 116 sec.

NExTEO is expected to give a ‘four-point’ improvement in punctuality on the two lines thanks to elimination of one third of the delays. It will also provide a 5% operating margin and possibly a gain in travel time.

The project is to be managed jointly by SNCF Réseau, SNCF Voyageurs and RATP. The investment in wayside equipment is costed at €967m, 70% of which will be funded by Île-de-France and 30% by central government, with the operators contributing €35m.

The relevant signalling control centres will be modernised, with much of the work concentrated at Villeneuve-Saint-Georges Poste 4, which was modified in March 2024 in preparation for the arrival of NExTEO. Fitting the trainsets will cost €239m, which will be funded entirely by Île-de-France-Mobilités.

Framework contract

A framework contract worth €500m to fit lines B and D was awarded in November 2023 to Alstom, which will base the system on its Urbalis CBTC equipment. NexTEO will not be fitted on the less-densely used sections operated by SNCF Reseau. These are currently equipped with a mix of KVB, KVBP and KCVP train protection systems; once these become life-expired, they will be replaced by ERTMS.

On the outer parts of the RATP sections, from Paris-Nord to Bourg-la-Reine, the equipment will be identical to that fitted to the SNCF Réseau sections and will be termed NExTEO Type 1. From Bourg-la-Reine to Robinson and St-Rémy-les-Chevreuse, where the traffic flows will be less intensive, RATP will fit NExTEO Type 2. This will provide an automatic driving facility, but with lineside signalling retained. This will minimise the interfaces between different signalling systems. Automated driving is expected to reduce the number of minor operating incidents and improve the robustness of the network.

Both ATS+ and SAE+ (Système d’Aide a l’Exploitation) could be deployed to complement NExTEO on both the SNCF Réseau and RATP routes. In addition, a centralised control facility will be built to regulate traffic on both lines, with joint operation by SNCF and RATP.

Among the many challenges ahead is the authorisation and approvals process. The sections of route operated by SNCF Réseau must meet TSI requirements, which in France are governed by government Decree 2019-525. EPSF will need to issue an Authority to Operate for all SNCF Réseau sections. For those parts of the route run by RATP, the Authority to Operate will be provided by the Prefet of the Île-de-France région under instruction from the Direction Régionale et Interdépartementale de l’Environnement, de l’Aménagement et des Transports. This is because the relevant sections are considered to be non-interoperable under Decree 2017-440.