ERTMS is the European Rail Traffic Management System. It is a safe system that offers train control by supervising the speed at any moment, thanks to track and train data. ERTMS is among the key elements to enhance rail interoperability within Europe and make it attractive to users. Let’s discover ERTMS, its history, concepts, blind spots, and possible evolutions.
Last revision : 08/01/2023
- ERTMS in-depth
- ERTMS/ETCS : european train control
- ERTMS/ATO : the future autonomous train
The need : to protect the trains
According to French railway safety agency EPSF , following dangers exist in railway operations:
- train collisions
- obstacle collision
To mitigate some of these dangers, railway companies started to implement signals along the tracks, that the train driver must respect at any time. However, this revealed itself to be insufficient. Indeed, the attention of the train driver is not guaranteed at any moment.
Hence, warning systems were installed to make sure that the train driver is aware of a closed signal. The French system crocodile is one of them and was invented in 1872 . It is composed of a piece of metal at the foot of the signal, and a brush on the locomotive. When the train passes the closed signal, the brush fetches an electrical signal on the piece of metal, and a warning sound is provided to the train driver. Then, the driver has a few seconds to acknowledge the warning to avoid an emergency braking.
The German system PZB has been developed in the 1930’s  and has three main functions:
- to apply the emergency brake if a train passes a red signal,
- to monitor the speed of the train and that is does not exceeds the maximal speed of the line,
- to monitor the acknowledgment of the train driver when passing a yellow signal.
The PZB is one of the many devices called ATP (Automatic Train Protection).
In France, the crocodile was not enough to prevent railway accident. In the 1980’s SNCF decided to protect the trains for overspeed and the crossing of red protection signals, with a new ATP: KVB . The speed control via balises (KVB: contrôle de vitesse par balises) is a mandatory equipment on a train in 2023, to get the homologation for France. 
Onboard computing unit UEVAL (Unité d’EVALuation) of French ATP KVB.
Crédit : Bastian SIMONI
National ATPs and the interoperability issue
France started to equip its network with KVB in the 1980’s. Other European countries also deployed ATPs, helped by their national industry. This quickly led to the situation where a lot of countries deployed specific and proprietary train protection system, incompatible between them. Consequently, international trains must be equipped with all ATP systems required in the countries they are homologated for.
The most famous example is the high-speed train Thalys, able to run in France, Germany, Belgium, and the Netherlands. For that, this train must be equipped with following ATPs: crocodile, KVB, TVM (France), PZB, LZB (Germany), TBL (Belgium), ATB (Netherlands).
All these different ATPs require volume in the train (for onboard computers, antennas). The train driver must be trained to use of these different systems, making his job more complex. Furthermore, a train equipped with multiple ATPs is much more complex and costly on its entire lifetime, because of maintenance, obsolescence management.
For railway undertakings, this a technical and economic burden, and a clear limit to European railway interoperability. Hence, the idea to create a unique train protection system within Europe emerged in the late 80’s, and lead to ERTMS: European Rail Traffic Management System.
In 2023, national ATPs are obsolete. They are named legacy systems, or class B systems , and must be replaced by ERTMS.
Thalys PBKA – By Mauritsvink, CC BY 3.0
Towards the creation of an interoperable ATP : ERTMS
The existence of more than 20 different ATPs in Europe became a clear obstacle to interoperability and fluidity in international railway operations. Thus, the discussion about an interoperable ATP started in the late 80’s, and the European Commission joined it in 1995, to define a development strategy for ERTMS. Some railway signalling companies joined forces within UNISIG, to write the specifications of the system in 1998 .
ERTMS is divided into three components :
- ETCS : European Train Control System. This is the European ATP, that will replace all legacy systems in the future,
- GSM-R : Global System for Mobile communications – Railway. This is a radio-based communication system, using 2G technology, so to provide voice and data service between the onboard and the trackside. GSM-R being developed on 2G technology, obsolete in 2030, it will be progressively replaced by its successor : FRMCS (Future Railway Mobile Communication System).
- ATO : Automatic Train Protection. This is an autopilot, enabling automatic traction and braking in presence of the driver.
UNISIG is a consortium regrouping some railway signalling suppliers. It has been founded in 1998, to start writing the ERTMS/ETCS specifications. In 2023, UNISIG’s mission is to develop, maintain and update the ERTMS/ETCS specifications with the European Railway Agency (ERA).
It’s members are : ALSTOM, AZD Praha, CAF, Hitachi Rail, MERMEC, Siemens, Thales, Progress Rail Signalling and MERMEC STE. 
ERTMS Users Group
The ERTMS users group is an association regrouping infrastructure managers and railway undertakings that make important investments into ERTMS/ETCS (> 250 million EUR).
The association proposes a discussion platform between the ERTMS users, to provide feedback and return of experience. It also enables the users to align their point of view and consolidates their proposals, for discussion with UNISIG and ERA. 
European Railway Agency (ERA)
ERA has been founded in 2004 and is an Agency of the European Union. Its role is to define the technical and juridical frame to result into a Single European Railway Area (SERA). The Agency works for a harmonized approach to railway safety by supporting ERTMS.  Henceforth, ERA acts as a System Authority for ERTMS. 
ERTMS/ETCS : european train control
The starting point is the Movement Authority : the permission for a train to run at a given speed and up to a given point of the track (End of Authority).
In the first days of the railways, the Movement Authority was a hand signal given by a trackside personnel, then by mechanical and light signals implemented along the tracks. Some class B systems dedicated to high-speed like TVM (France) or LZB (Germany) display the Movement Authority directly in the cabin of the driver. This is CAB-Signalling.
The objective of ERTMS/ETCS is to transmit the Movement Authority from the trackside to the onboard, so that it can be displayed to the driver via a dedicated screen. The onboard controls that the actions of the driver are compatible with the provided Movement Authority. An autopilot can also be used, this is ATO over ETCS.
To realize its functions, ERTMS/ETCS is composed of trackside and onboard subsystems.
The onboard contains all elements necessary to :
- display the Movement Authority to the driver,
- apply the emergency braking in case of overspeed,
- apply the emergency braking in case of overshoot of the End of Authority,
- train positioning on the track plan.
The onboard subsystems rely on a safety computer: the European Vital Computer (EVC), and peripherals:
- the display in the cabin: Driver Machine Interface (DMI),
- antennas to fetch the signalling information from the trackside,
- the odometer, to determine the position of the train on the track plan.
To get the complete view on the onboard components, go to Architecture.
The trackside subsystem is composed of all elements necessary to elaborate the Movement Authority, and to transmit it to the onboard.
The fundamental elements are the following:
- Lineside Electronic Units (LEU), coders that retrieve the signalling information from the lineside signal or the interlocking, convert it into ERTMS/ETCS compliant information, and transmit it to Eurobalises,
- Eurobalises, that transmit the ERTMS/ETCS compliant information (fixed or switchable via LEU) at the passing of the train,
- Radio Block Centre (RBC), that retrieves the signalling information from the interlocking, converts it into ERTMS/ETCS compliant information, and transmits via radio to the onboard units.
To get the complete view on the trackside components, go to Architecture.
The onboard ERTMS/ETCS determines where the train is located on the track plan. For that, the railway track must be studied to get its specifications and topology. A track survey can be done, with a measurement train equipped with sensors.
The onboard locates the train on the track plan with an odometer. The problem is that the current odometry systems creates a cumulative error, that must be regularly reset. For that, Eurobalises are periodically installed on the track, and integrated into the track description. Thus, when the train passes the Eurobalise, the onboard relocates itself immediately, as the location of this balise on the track plan is perfectly known. In this way, the odometry error is reset.
In 2023, the replacement of this odometry system by satellite-based systems and Inertial Measurement Units (IMU) is investigated. These new technologies could reduce the amount of relocation balises on the track. The ERTMS users group has issued in 2022 a specification for LOC-OB. 
As defined in , the different ERTMS/ETCS application levels (short: levels) are a way to express the possible operating relationships between track and train. Level definitions are related to the trackside equipment used, to the way trackside information reaches the onboard units and to which functions are processed in the trackside and in the on-board equipment respectively.
Different levels have been defined to allow each individual railway administration to select the appropriate ERTMS/ETCS application trackside, according to their strategies, to their trackside infrastructure and to the required performance. Furthermore, the different application levels permit the interfacing of individual signalling systems and train control systems to ERTMS/ETCS.
ERTMS/ETCS can be configured to operate in one of the following application levels:
- ERTMS/ETCS Level 0 : train equipped with ERTMS/ETCS operating on a line not equipped with any train control system (ERTMS/ETCS or national system) or on a line equipped with ERTMS/ETCS and/or national system(s) but operation under their supervision is currently not possible.
- ERTMS/ETCS Level NTC : train equipped with ERTMS/ETCS operating on a line equipped with a national system.
- ERTMS/ETCS Application Level 1 with or without infill transmission : train equipped with ERTMS/ETCS operating on a line equipped with Eurobalises and optionally Euroloop or Radio infill.
- ERTMS/ETCS Application Level 2 : train equipped with ERTMS/ETCS operating on a line controlled by a Radio Block Centre and equipped with Eurobalises and Euroradio with train position and train integrity proving performed by the trackside.
- ERTMS/ETCS Application Level 3 : similar to level 2 but with train position and train integrity supervision based on information received from the train.
Level NTC : backward compatibility with Class B systems
The replacement of class B systems with ERTMS/ETCS must be done in a synchronized way between the tracks and the trains. This is an important migration challenge. To facilitate it, the ERTMS/ETCS onboard has been conceived to coexist with class B systems. This is possible thanks to the level NTC (National Train Control).
In level NTC, the train is equipped with ERTMS/ETCS, and the track is still equipped with the class B system. The onboard part of the class B system can be connected to the onboard ERTMS/ETCS, so to benefit from resources like the cabin screen or the odometry.
This connection is made via a module called STM (Specific Transmission Module), and the interface between STM and ERTMS/ETCS onboard is standard. The STM is responsible of the train supervision in level NTC.
Different class B systems within Europe have been adapted for the use with STM and ERTMS/ETCS.
Trackside and onboard ERTMS/ETCS components, alongside the legacy system and STM. Credit: MERMEC
The integration of class B systems within the ERTMS/ETCS ecosystem via STMs enables a better coexistence of the systems onboard. ERTMS/ETCS realizes automatic transitions between ETCS levels and level NTC with transition Eurobalises installed on the track. The driver interacts with ERTMS/ETCS and the STM via the unique driver-machine interface in the cabin. The human factors are therefore enhanced.
When they were designed, STMs were seen as a convenient migration tool. Today, they have become a source of complexity and an important burden for the transition to ERTMS/ETCS. The key innovation consists in getting rid of hardware STMs, and to move to software STMs directly hosted within the European Vital Computer in a dedicated runtime environment. Then, as a smartphone on which it is possible to install various applications, the EVC could host different STMs as Apps. The onboard EVC would become a Multistandard platform, a fundamental tool for the migration to ERTMS/ETCS.
The Multistandard EVC is proposed by signalling suppliers ALSTOM and The Signalling Company .
Onboard computing unit ALSTOM CCP2, revealed during Innotrans 2022. Credit: Bastian SIMONI
This onboard unit can host ERTMS/ETCS application, Automatic Train Operation (ATO) and class B systems as Apps (Multistandard).
Level 1 : the upgrade based on existing systems
As defined in , ERTMS/ETCS Level 1 is a spot transmission-based train control system to be used as overlay on an underlying signalling system. Movement Authorities are generated trackside and transmitted to the onboard via Eurobalises, fed by Lineside Electronic Units (LEU).
Level 1 provides a continuous speed supervision system, which also protects against overrun of the authority.
Train detection and train integrity supervision are performed by the trackside equipment of the underlying signalling system (interlocking, track circuits, axle counters, etc) and are outside the scope of ERTMS/ETCS.
A fundamental step of an ERTMS/ETCS Level 1 project, is the application engineering. Indeed, this step consists in studying the line on which Level 1 will be deployed, and to create all necessary ERTMS/ETCS compliant data.
These data are put into the coders (LEU) installed on the line. The coders can be connected to the interlockings or the lineside signals. The LEUs are programmed with the application engineering data, with all ERTMS/ETCS data associated to the interlocking or the signal they are connected to. The LEU transmits to the Eurobalise the proper ERTMS/ETCS data, related to the state of the interlocking or the signal, as defined during the application engineering process.
This scheme represents the flow of signalling information, starting from the signal or interlocking, retrieved by a LEU that sends the corresponding ERTMS/ETCS data to the Eurobalise, fetched by the onboard Balise Transmission Module (BTM) and EVC, and then displayed to the driver on the Driver Machine Interface. Credit : MERMEC
Level 1 when approaching a red signal.
Representation of Level 1 : red signal approach, only equipped with Eurobalises. Credit : ertms.net
The signal is closed (red), the state is retrieved by the LEU, that associates this state with ERTMS/ETCS compliant data defined during the application engineering phase. If the signal remains red, the driver must not pass the Eurobalise. By doing so, the onboard would automatically apply the emergency brake. The driver must wait until the signal is open to cross the Eurobalise. The signal LEU would have modified the ERTMS/ETCS data in the balise, and the onboard would receive its new Movement Authority.
Level 1 being a spot transmission system, existing lineside signalling must be kept. It is not a 100% CAB-signalling system. Semi-continuous infill devices can enable CAB-signalling and then the lineside signals can be suppressed. These devices are:
- Euroloops, inductive loops placed before the Eurobalise,
- Radio Infill Units (RIU), a local radio transmission device, placed at the foot of the signal.
Representation of Level 1 : red signal approach, equipped with Euroloop. Credit ertms.net
If the signal is equipped with infill devices, like Euroloop or RIU, then the onboard receives immediately the new Movement Authority when the signal is open.
On the scheme, the black line represents an inductive loop Euroloop, that transmits continuously the ERTMS/ETCS data provided by the LEU and associated to the state of the signal. Although the train antenna has not yet crossed the balise, it is above the loop and receives the refreshed Movement Authority immediately when the signal state changes.
Level 2 : radio signalling
As defined in , ERTMS/ETCS Level 2 is a radio-based train control system which is used as an overlay on an underlying signalling system. Movement Authorities are generated trackside and are transmitted to the train via Euroradio. For that, a Radio Block Centre is connected to the existing or upgraded interlocking.
ERTMS/ETCS Level 2 provides a continuous speed supervision system, which also protects against overrun of the authority.
Train detection and train integrity supervision are performed by the trackside equipment of the underlying signalling system (interlocking, track circuits, axle counters, etc) and are outside the scope of ERTMS/ETCS.
Level 2 is based on Euroradio for track to train communication and on Eurobalises as spot transmission devices mainly for location referencing.
ERTMS/ETCS Level 2, credit MERMEC.
In this example, lineside signalling is suppressed as signalling is 100% digital.
ERTMS/ETCS Level 2. Crédit : ertms.net
Level 3 : lightweight infrastructure
As defined in , ERTMS/ETCS Level 3 is a radio-based train control system. Movement Authorities are generated trackside and are transmitted to the train via Euroradio.
ERTMS/ETCS Level 3 provides a continuous speed supervision system, which also protects against the overrun of the authority.
Train position and train integrity supervision are performed by the trackside Radio Block Centre in cooperation with the onboard (which sends position reports and train integrity information).
Level 3 is based on Euroradio for track to train communication and on Eurobalises as spot transmission devices mainly for location referencing.
ERTMS/ETCS Level 3. Crédit : ertms.net
Hybrid Level 3
Hybrid Level 3 is a specific concept for the implementation of Level 3, defined by the ERTMS Users Group . The main characteristic of the concept is that it uses fixed virtual blocks for the separation of trains which are fitted with a train integrity monitoring system (TIMS), while a limited installation of trackside train detection is used for the separation of trains without TIMS, as well as for the handling of degraded situations.
ERTMS/ETCS Level 3 relies on preconditions, that are difficult to match in 2023:
- In Level 3, the separation of trains is managed by ERTMS/ETCS, thanks to the position and integrity reports of the trains. Each train must be equipped with a TIMS, and the use of TIMS on variable composition trains (especially freight) is complex,
- The total absence of train detection devices trackside supposes that ERTMS/ETCS Level 3 knows at any moment the position and integrity of all trains within the area under its responsibility. In practice, this is not always possible:
- If the train is moved via operational procedures, not under the supervision of ERTMS/ETCS,
- If the radio communication fails.
Hybrid Level 3 is an interesting answer to the challenges of Level 3. Indeed, keeping a trackside train detection system enables:
- The compatibility with freight trains, for which the use of TIMS is complex today. It will be facilitated in the future with the Digital Automatic Coupling (DAC),
- The management of trains disconnected from the trackside hybrid level 3, as they will still be detected by the train detection system. This is particularly interesting for movements via operational procedures, or in case of rebooting of the trackside in case of a crash.
In its concept, Hybrid Level 3 does not use virtual moving block, but virtual fixed block. This is not a fundamental need of the concept. It is just for pragmatic reasons. In comparison to moving blocks, fixed virtual blocks have in several implementations less impact on the existing trackside systems such as the RBC, interlocking and traffic control centre as well as on the operational procedures. By reducing the length of the virtual blocks, the performance can be similar to moving blocks, which means that the performance benefit is not compromised.
Architecture of ERTMS/ETCS
When ERTMS/ETCS was specified, interoperability was a fundamental aspect. The objective was to enable smooth border crossing, but also the possibility to use onboard ERTMS/ETCS from supplier A in combination with a trackside from supplier B.
The specification of ERTMS/ETCS defines the system architecture, with plug’n’play interfaces FFFIS (Form-Fit Function Interface Specification).
ERTMS/ETCS architecture according to SUBSET-026 §2 version 3.6.0
ERTMS/ETCS architecture as defined in .
The onboard ERTMS/ETCS contains :
- BIU/TIU : the module that interfaces with the train, notably to apply the emergency braking,
- DMI : Driver Machine Interface,
- Juridical data : the interface function with the onboard recording device,
- BTM : Balise Transmission Module, the device that retrieves the information from the Eurobalises,
- LTM : Loop Transmission Module, the device that retrieves the information from the Euroloops,
- Euroradio : the GSM-R modem.
The trackside ERTMS/ETCS contains :
- RIU : Radio Infill Unit, providing signalling information in advance as regard to the next main signal in the train running direction,
- RBC : Radio Block Centre, a computer-based system that elaborates messages to be sent to the train on basis of information received from external signalling systems (e.g. interlocking) and on basis of information exchanged with the onboard subsystems,
- Balises : a transmission device that can send telegrams to the onboard subsystem,
- Euroloops : inductive loops providing signalling information in advance as regard to the next main signal in the train running direction.
Trackside elements around the ERTMS/ETCS scope (in green) are :
- LEU/interlocking : the lineside electronic units are electronic devices, that generate telegrams to be sent by balises, on basis of information received from external trackside systems,
- KMC : the role of the KMC is to manage the cryptographic keys, which are used to secure the Euroradio communications between ERTMS/ETCS entities,
- PKI : the role of the PKI is to manage and distribute digital certificates, so to allow a secure online distribution of cryptographic keys between KMCs and from a KMC to the ERTMS/ETCS entities,
- Control Centre
- National System : existing class B systems
The blind spot : the long-lasting rollout of ERTMS/ETCS
Since its conception in the 1990’s and its first deployments, the rollout rhythm of ERTMS/ETCS is slow. Although it is meant to replace existing class B systems, and facilitate interoperability, a massive rollout of ERTMS/ETCS is still awaited.
Deployment data (stand 12/2021), credit ertms.net
The German association Allianz Pro Schiene shared in November 2022, helped with SCI Verkehr data, its comprehension of the ERTMS/ETCS rollout plans in some European countries. We can discover that 20% of French and German networks will be covered by ERTMS/ETCS in 2030. Projection of 90% of the network is expected in 2040.
In real terms, this means that from now on to 2040 :
- Class B systems will remain inescapable: software STMs and Multistandard platforms will be a must,
- The ATO over ETCS solution, as defined in 2022, is not massively usable until 2040. Indeed, automatic traction and braking, in presence of a driver, needs ERTMS/ETCS installed both onboard and trackside!
The opportunity : decouple trackside and onboard fitment
ERTMS/ETCS rollout faces the chicken and egg problem. For the complete system to work, ERTMS/ETCS must be installed and the tracks but also on the rolling stock.
For the rollout to be efficient, infrastructure managers and railway undertakings must have shared interests to deploy in a synchronised way. Reality is much more complex.
Infrastructure managers have already a lot to do, with network regeneration and maintenance, and with the budget they are granted. Railway undertakings, especially historical ones, do not see the interest in fitting their existing fleet with ERTMS/ETCS if the class B system does the job. These class B systems are also an entry barrier into a market for newcomers. This has been pointed out by the French Transportation Regulation Authority in its report on rail liberalisation edition 2022 .
Deployment of ERTMS/ETCS can be done because of constraints or opportunities:
- Economic ones: to benefit from European funding, especially on newbuild lines,
- Technical ones: if the class B suppliers does not want to maintain it anymore.
Instead of enduring the rollout of ERTMS/ETCS, it could be possible to turn it into an advantage for railway undertakings. Even though the trackside is not yet fitted with ERTMS/ETCS, there should be a good benefit to install ERTMS/ETCS onboard.
This benefit is simple. It is to feed the onboard ETCS with a signalling information that already exists: lineside signalling. It is the principle of Level 1, with the LEU that retrieves the state of the signal and transmits the according ERTMS/ETCS data to the balise. Instead of deploying equipment along the track (LEU and balises), why not making this signal state acquisition directly onboard?
With an onboard ETCS, capable to perceive the signal state and to associate it with its ERTMS/ETCS compliant information, it is not necessary to install equipment on the track anymore. The decoupling is realized.
Towards a virtual Level 1
When ERTMS/ETCS Level 1 was specified in the 1990’s, the state of art could not enable the retrieving of the signal state directly onboard. Indeed: there were no cameras, computer vision algorithms for that. The signal state acquisition was then made trackside, via coders connected to balises. Furthermore, cumulative errors of the odometry were reset with relocation balises. Fitting the trackside was inevitable.
Today, the context is different:
- Progress was made in cameras, computer vision and onboard computing power: this enables the acquisition of the signal state onboard via visual perception,
- Onboard navigation, satellite-based and helped with inertial measurement units suggest the location on the track plan with limited errors and a reduced amount of relocation balises.
This means that Level 1 defined in the 1990’s can be virtualized with technologies from the 2020’s: computer vision, onboard navigation, and digital map.
Benefits of ETCS Level 1 virtual are simple:
- Providing visual Automatic Train Protection, potentially enhancing the safety of railway operations on lines fitted with a legacy ATP, and on lines not equipped with a legacy ATP at all.
- Enabling the use of automatic driving, ATO over ETCS, on all lines equipped with lineside signalling. This is a huge benefit and reason for railway undertakings to equip existing rolling stock with ERTMS/ETCS. Specially to benefit from energy savings in a tense energy context in Europe in 2023.
ETCS Level 1 virtual could be a game-changer by initiating an important onboard rollout of ERTMS/ETCS. Railway undertakings would have direct benefits by doing so. This concept is also a migration tool towards ERTMS/ETCS Level 3 and ATO up to GoA4. Indeed, the trains would already been equipped with onboard ERTMS/ETCS, Localization, Perception, and a radio module (GSM-R/FRMCS). All these elements are necessary for Level 3 and/or ATO up to GoA4.
ERTMS/ATO: the future autonomous train
The autonomous train is currently being specified at European level , and prototyped and tested by various projects . It is seen as a tool to enhance the capacity and flexibility of railway networks and operations.
What is an autonomous train ?
ERTMS/ETCS is the central part of the autonomous train. Indeed, it is essential within the ATO over ETCS solution, enabling automatic traction and braking in presence of a driver. For ATO up to GoA4, ERTMS/ETCS will supervise the proper application of the rules in case of incident.
ATO over ETCS
The autopilot option of ERTMS/ETCS has been specified during the 2010 decade by users and suppliers. This became ATO over ETCS. The solution will be part of the Technical Specification for Interoperability – Control Command Signalling 2022. It is already in use like in Hamburg on line S21. 
ATO over ETCS (GoA2) architecture.
ATO over ETCS
ATO over ETCS-Onboard
The major problem of ATO over ETCS, is its dependence to ERTMS/ETCS onboard and trackside. In 2023, it is not possible to use ATO over ETCS, on lines not equipped with ERTMS/ETCS. As we have seen in the ERTMS/ETCS rollout previsions, the massive use of ATO over ETCS is not for tomorrow.
This important constraint can limit the adoption of standard solution ATO over ETCS. The benefits of reduced energy consumption, specially in 2023 with a tense energy context in Europe, are voided. Therefore, migration solutions are imperative.
ERTMS/ETCS Level 1 virtual is one of them. Besides protecting the train (visual ATP), it opens the use of ATO over ETCS without any modification, on all lines equipped with lineside signalling. Railway undertakings have an immediate and clear benefit to fit their trains with ERTMS/ETCS. This is ATO over ETCS-Onboard.
ATO over Lineside Signalling
As migration solutions for ATO over ETCS were seen as a must by the European Commission, investigations were launched within a new project of Shift2Rail program: Technologies Supporting Migration to ATO over ETCS. Is gather signalling suppliers and users, to define a concept enabling the use of ERTMS/ETCS via visual perception of existing lineside signals. 
ATO up to GoA4
Today, ERTMS/ETCS onboard supervises only the good respect of the Movement Authority by the driver or ATO. If the driver or ATO does not respect the Movement Authority, then ETCS applies the emergency braking. With ATO up to GoA4, a new ERTMS/ETCS onboard would fuse the signalling Movement Authority, with restrictions provided by the module in charge of managing the incidents. Then, the onboard would apply the most restrictive information and communication it to ATO so that it adapts its driving, or directly apply the emergency brake.
Standardisation activities are still ongoing within the X2RAIL4 project of Shift2Rail program.
20 years after being specified, ERTMS/ETCS remains poorly rolled out, both trackside and onboard.
The slow progression of ERTMS/ETCS installation maintains the class B systems, obsolete and proprietary. They are an important entry barrier into a market, and a pain for railway undertaking having international operations.
Although rail is without any doubt a solution for a climate-friendly mobility, important progress must be made to encourage the installation of ERTMS/ETCS and rise the modal share of rail.
It is pragmatic to think that class B systems will still be there in the next 20 years. This constraint must be turned into an opportunity. Software STMs and Multistandard onboard platforms are the future and a key migration tool.
Furthermore, technological breakthroughs enable to reconsider ERTMS/ETCS principles stated in the 1990’s. It is the case for ETCS Level 1 virtual, based on computer vision, onboard navigation and digital map.
The autopilot option, ATO over ETCS, is specified into the Technical Specification for Interoperability – Control Command and Signalling edition 2022. This function, that has been used for many years in the Urban, makes the rollout of ERTMS/ETCS more attractive and enables Mass Transit on the Mainlines.
ERTMS/ETCS will play a key role within the future autonomous train. Besides supervising the respect of signalling Movement Authority, it will supervise the respect of the rules. This major evolution will be discussed and specified in the European working groups for the next years, so to update the ERTMS/ETCS specification SUBSET-026.
To go further
ERTMS/ETCS specifications : available in the library.
ATO over ETCS (GoA2) specifications : available in the library.
ATO up to GoA4 specifications : available in the library.
Cover picture credit : ALSTOM
 Railway Signalling & Automation, Volume 3, W. Schön, G. Larraufie, G. Moëns, J. Poré.
 SUBSET-026, version 3.6.0