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ATO over Lineside Signalling


The standard solution ATO over ETCS is an extension of the european signalling system ETCS, so to bring automatic driving. However, the number of tracks equipped with ETCS remains limited. Is it still possible to use the standard solution, on tracks only equipped with lineside signalling ?

If you are not familiar with ATO over ETCS, I invite you to read the associated article.

ATO over ETCS : the european standard solution

The standard solution ATO over ETCS has been specified by a european working group, within the Shift2Rail public-private partnership. This specification activity had following objectives :

  • To create the ATO add-on for the european signalling system ETCS,
  • To converge towards a standard, interoperable and interchangeable solution,
  • To become the unique solution for ATO Mainline use cases within the european Union, by integrating the 2022 revision of the Technical Specification for Interoperability – Control Command and Signalling.

Hence, the standard solution ATO over ETCS is to be understood as an ETCS extension. Its System Requirements Specification, SUBSET-125, is precising that the use of ATO over ETCS without an onboard ETCS unit is out of scope. This means, that ATO over ETCS is not specified for a use case with class B ATP systems.

The ATO/ATP interface of the ATO over ETCS solution, SUBSET-130, is plug’n’play : FFFIS (Fit Form Function Interface Specification).

ATO over ETCS architecture

ETCS : a long and costly deployment 

Since its conception in the 1990s, and its first deployment in the 2000s, adoption of ETCS remains limited. Although ETCS is supposed to replace class B systems, and facilitate interoperability, ETCS has not been installed with the same energy within the european countries.

In France, in 2021, 7474 kilometers of track are currently contracted for a ETCS deployment. On a 35 000 kilometers wide network, this represents 21%. Furthermore, Luc Lallemand, Head of SNCF RESEAU (the French infrastructure manager), confirmed that the installation of ERTMS/ETCS is not a priority.

A l’horizon de 40 ou 50 ans, tous les réseaux en Europe seront équipés de ERTMS, mais aujourd’hui sur le réseau français, les priorités sont ailleurs.

Within 40 or 50 years, all european railway networks will be equipped with ERTMS. However, today for the french network, priorities are in other fields.

Ville, Rail et Transports, edition 646. Picture : Aurore Baron

Luc Lallemand

CEO, SNCF Réseau

ERTMS figures

As per specification, the standard solution ATO over ETCS is linked to :

  • the equipment of rolling stock with ETCS
  • the installation of ETCS on tracks

Railway undertakings could choose using a proprietary ATO solution, linked to their class B system. However this is leading to :

  • an extended lifetime of the class B system, considered as legacy and obsolete by the UE. Class B systems must be replaced by ETCS,
  • the impossibility to cross borders with the ATO engaged (unless by developping a system capable of interfacing with N class B systems, which is the opposite of the ETCS spirit),
  • a solution incompatible with future TSI CCS 2022, which will become the law once adopted by UE state members.

The challenge relies in using the standard solution ATO over ETCS and its standard interfaces, without ETCS.

The will of operators to use automatic driving

Some railway undertakings do not want to wait for the ETCS to be installed, to use automatic driving. SNCF is considering automatic driving on the mainline network for freight and regional passenger trains for 2025. Nederlandse Spoorwegen is experimenting adapted ATO over ETCS to run on their national systems. Railway undertakings are exploring for migration solutions, to launch automatic driving as soon as possible.

A relevant migration solution is based on the standard solution ATO over ETCS : the building blocks and their interfaces. Parts of ETCS not already deployed are emulated thanks to adapters. The day ETCS is installed, only the adapters are removed. The rest of the system remains untouched.

ATO over ETCS (over ATB) : the Dutch experiment


The Dutch railway undertaking NS, has experienced adapted ATO over ETCS between april 2020 and september 2021. The experiment was done with suppliers CAF Signalling and Ricardo Rail. The objective was to adapt the standard solution ATO over ETCS, and use it on a train and a track not equipped with ETCS. [1]

As explained in the article ATO over ETCS, ETCS is giving to ATO the signalling information that must be respected. If ATO does not, then ETCS applies the emergency braking. In the experiment conducted by NS, there is no ETCS.

ATO over ETCS without ETCS

The absence of ETCS is balanced by a dedicated adapter : ATO-ADAPTER. This device fetches all signalling information that is provided to it, whatever the format, and converts it into SUBSET-130 compatible information. NS chose to use three different systems to feed the ATO-ADAPTER :

  • Lineside signalling
  • Class B system ATB
  • Interlocking and block occupancy detection systems

    ATO over ETCS (over ATB) in depth

    The starting point being the mission to be operated, ATO-OB receives from ATO-TS Journey Profiles and Segment Profiles. Standard interface SUBSET-126 is used for it, and ATO-TS fetches the timetable for Prorail’s timetable system Donna.

    New information is added in this interface between Trackside and Onboard : the expected aspect of lineside signals. This extra information is based on SFERA standard. SFERA is a working group within UIC, to complete SUBSET-126 with additional data, to propose a driving assistance system. [2]

    Routelint interlocking system has been connected to the system, to retrieve the occupancy state of the blocks. With this information, the system is capable of expecting the state that lineside signalling will present (expected signal aspect).

    These data are transmitted regularly from trackside to onboard. The ATO-ADAPTER emulates the onboard ETCS unit, and feeds ATO with signalling information compliant to SUBSET-130. However, this adapter does not provide any safety function. ATB class B system is in charge of it, and applies the braking, if ATO does not respect its warning and protection curves. To avoid any intervention of ATB, the ATO-ADAPTER has to take into account ATB specificities, and to adapt them within SUBSET-130 interface.

    Onboard class B system ATB is therefore connected to the ATO-ADAPTER, and provides to it the ATB signalling information (ATB codes). A localization device is also connected (WITbox), enabling ATO-ADAPTER to localize the train on the trackplan. Thanks to this localization information, and the lineside signals static data provided by trackside (signals, expected signal aspect), perception system PER searches within the physical environment the next signal, and provides its state to ATO-ADAPTER.

    The driver confirms this signal state on a human-machine interface. Then, ATO-ADAPTER possesses all relevant information, converts it into SUBSET-130 compliant data, and transmits it to ATO.

    Thanks to this adapter, the onboard ATO remains compliant with SUBSET-125 and standard ATP/ATO interface SUBSET-130. ETCS-related data within the SUBSET-130 are reproduced by ATO-ADAPTER.

    ATO over ATB

    To synthesize the main principles of the system :

    • The mission to be operated is completed with additionnal information, provided by the infrastructure manager :
      • Location of lineside signals and their characteristics
      • Block occupancy state
    • Signalling information normally provided by an onboard ETCS unit is reproduced by an ATO-ADAPTER, which fuses :
      • Data coming from class B system ATB
      • The expected signal aspect provided by trackside, and confirmed by the driver on a human-machine interface

    ATO-ADAPTER replaces the ETCS onboard unit, and adapts the different signalling information it can get, into SUBSET-130 compliant information. Furthermore, the adapter takes into account ATB specificities, so to make the ATO drive the train without any ATB intervention.

    Safe Trainmovement

    Advantages and blind spots of the NS experimentation

    This experiment conducted by NS can be considered as a migration solution, as it reused the standard solution ATO over ETCS with adapters to make it compatible with existing systems. The interfaces around the onboard ATO were therefore respected (SUBSET and SFERA).

    However, some aspects can be considered as problematic :

    • The link with class B system ATB can encourage extending its lifetime. ATB is considered as legacy by the UE and must be replaced by ETCS.
    • The use of static data from the infrastructure manager : what is the availability and quality of the data ?

      SNCF’s two pilot projects

      SNCF wants to launch mainline automatic driving for freight and regional passenger trains starting 2025. To achieve this, SNCF and industrial partners are exploring the feasibility to use ATO over ETCS standard solution with adapters, to operate over lineside signalling. Innovation projects are Autonomous Freight Train and Regional Passenger Train.

      Both projects are meant to confirm the feasibility of automatic driving with ATO over ETCS, on lines not yet ETCS-equipped, and using only lineside signalling. SNCF already investigated in the past the detection of lineside signalling within project TAS (Transport terrestre Autonome en Sécurité dans son environnement) [3].

      In 2022, Autonomous Freight Train project achieved automatic driving using adapted ATO over ETCS solution, over french lineside signalling BAL.

      ATO over ETCS-Onboard

      A necessary migration period

      As we have seen in the beginning of the article, ETCS-equipped kilometers of track remain limited. The deployment dynamic is different in the UE states. This problem is delaying the adoption of standard solution ATO over ETCS.

      Investigations on SNCF and NS side highlighted the possibility to use ATO over ETCS with adapters, as migration solution.

      The European Commission saw an opportunity in there, to accelerate the adoption of ATO over ETCS. Hence, a new program within Shift2Rail was accepted in 2020 : Technologies Supporting Migration to ATO over ETCS. This program is lead by Alstom, and hosts SNCF as user and signalling suppliers. [4]

      The objective of this program is to propose new onboard ETCS functions, for using signalling information provided by a lineside signalling perception and interpretation device. Link to any class B system is forbidden.

      This project consists in decoupling ETCS-Onboard and ETCS-Trackside. This opens the way to use an ATO over ETCS-Onboard solution on any track equipped with lineside signalling.

      Lineside Signalling perception : a new approach of ETCS Level 1

      Today, onboard ETCS gets information from trackside by two different means:

      • Occasionally : via balises linked to a coder (LEU – Lineside Electronic Unit), that fetches the lineside signal state. These balises can be completed with anticipation devices (radio infill or euroloop). This is ETCS level 1.
      • Continuously : via a track-to-train radio link using euroradio, linking the onboard to the radio block center (RBC). This is ETCS level 2, where lineside signalling can be suppressed.

      To get an overview of the ETCS system, I recommend you to read SUBSET-026 chapter 2.

        MERMEC ETCS L1
        MERMEC ETCS L2
        ATO over ETCS

        Within the Technologies Supporting Migration to ATO over ETCS program, there are no balises nor RBC. The studied onboard ETCS has to get its trackside signalling data by another mean. Connection to an existing class B system is forbidden, as ETCS is meant to replace it !

        The alternative, that is well deployed, is lineside signalling : static and dynamic signals along the track. The evolved onboard ETCS would have a new interface, to a perception and interpretation device.

        This strategy has following advantages :

        • It makes national lineside signalling principles generic – as does current ETCS Level 1.
        • Feeding an onboard ETCS with lineside signalling information opens to train protection based on this new input. Onboard class B system can be decommissioned.
        • When sufficient rolling stock is equipped with this system, the railway undertaking gets influence towards the infrastructure manager, to incite him to deploy trackside ETCS.
        • The standard solution ATO over ETCS can be used without adapters.
        ATO over ETCS over Lineside Sig

        In ETCS Level 1, the state of the lineside signal is reproduced within a balise, via a coder : the LEU. The lineside signal state is country specific, and the signalling data transmitted from the balise to the onboard ETCS is standard. It is the goal of ETCS : to enable a train equipped with ETCS, to run on any track in Europe, equipped with ETCS. The application engineering, meant to establish all possible ETCS data for each signal state, is one of the most important steps in a ETCS Level 1 deployment.

        This philosophy can be applied here : the perception and interpretation system can be compared to a balise and a LEU in Level 1. Perception system searches within the physical environment the signals addressing the train, and transmits the perceived state to a signalling converter. This signalling converter adapts the signal state into ETCS standard signalling information, as would a LEU do. The application engineering process done for ETCS level 1 to configure the LEUs can be used to configure the signalling converter.

        To enhance system performance, the signalling converter gets the localization of the train on the trackplan, and signal static data (their location, frame).

        Onboard ETCS is then able to supervise the train, thanks to this new input, as it would do in Level 1. Additionally, the standard solution ATO over ETCS can be used for automatic driving.

        ETCS L1 virtual

        The certification challenge

        ETCS supervision means safety. Today, ETCS systems onboard and trackside are safety-related and certified. The starting assumptions of the studied system are following :

        • Trackplan static data are safety-related
        • The localization device is safety-related
        • The perception and conversion devices are safety-related

        To create and maintain the trackplan data is not something new : it is already done when ETCS is deployed and when the existing data is not accurate enough. However, for lineside signals, the difficulty might be in the obtention of their characteristics and the uncertainties associated.

        Suppliers will have the challenge to develop safety localization and perception devices.


        The standard solution ATO over ETCS is linked to the ETCS ATP as per specification. However, the long-lasting installation of ETCS on tracks is delaying the adoption of this standard solution. Meanwhile, ATO over ETCS will integrate the 2022 revision of the Technical Specification for Interoperability – Control Command and Signalling. Once adopted by UE state members, this TSI will impose ATO over ETCS as unique solution to grant interoperability and interchangeability.

        Some experiments were conducted by SNCF and NS, using standard solution ATO over ETCS with adapters. These trials highlighted the feasibility of migration solutions, before ETCS is installed. The European Commission saw in there an opportunity to boost the adoption of ATO over ETCS, and launched a new program with users and suppliers. The objective is to define new onboard ETCS functions, fed by lineside signalling perception and conversion devices. This concept can be understood as a new approach to ETCS Level 1 acquisition of lineside signals states, totally onboard. The project opens a decoupling between ETCS-Onboard and ETCS-Trackside, making possible ATO over ETCS-Onboard on any track equipped with lineside signalling.

        This concept present noticeable advantages :

        • Train protection with ETCS, based on lineside signalling perception and interpretation. Therefore, onboard class B system can be decommissioned.
        • As side effect, railway undertakings can be tempted to install this system instead of national systems/STMs to access a foreign network. The augmentation of ETCS-equipped rolling stock become a mean to invite the infrastructure manager to deploy ETCS trackside, especially level 3 when available.
        • Automatic driving using ATO over ETCS without any adapters is possible.

        Beyond the concept, the certification challenge is real. Suppliers will have the task to conceive and develop safety products.

        To (re)discover the standard solution ATO over ETCS, I invite you to read this article !

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