From Fixed Blocks to Moving Block: Unlocking Capacity on Existing Railway Infrastructure

by Bastian

Railway signalling is often presented as a safety system. This is true. The first role of signalling is to prevent unsafe train movements. It must prevent collisions, overspeeding, conflicting movements, derailments caused by wrongly positioned points, and movements into areas that are not safe for the train. But signalling is not only about safety.

Signalling also defines how close trains can run to each other. And because railway capacity strongly depends on how close trains can safely run, signalling is also a capacity architecture.

This is why the evolution from conventional fixed blocks to ETCS Level 2, Hybrid Train Detection and Moving Block is so important. It is not only a technical evolution of the European Train Control System. It is a progressive transformation of how the railway system knows where trains are, how it determines whether track is free, and how it calculates the authority given to each train.

The objective is simple to express, but difficult to implement: reduce the safe separation between trains without reducing safety.

This matters because railway infrastructure is increasingly expected to carry more traffic. Urbanisation, modal shift, saturated corridors, environmental constraints and the difficulty of building new lines all point in the same direction: existing railway infrastructure will have to be used more efficiently. New infrastructure remains necessary in many places. But it is expensive, slow to deliver and often difficult to accept locally.

This does not mean that signalling alone can solve the capacity problem. Capacity is a system property. It depends on signalling, braking performance, rolling stock, dwell times, timetable design, junctions, station layouts, traffic management, operational rules, degraded modes and passenger flows.

But signalling remains one of the key constraints. If the signalling system requires a long protected distance between two trains, the theoretical capacity of the line is limited. If the signalling system can safely reduce this distance, the capacity envelope of the infrastructure increases.

This article explores that evolution.

It starts from conventional fixed blocks and ETCS Level 2. It then explains Hybrid Train Detection, Moving Block, train integrity, train length, safe positioning, the Digital Register, freight migration, the interlocking question, the cost-benefit challenge, and finally why ERTMS/ATO becomes necessary to operate close to the new capacity envelope.

The central idea is this: HTD and Moving Block can create a higher capacity envelope. ERTMS/ATO can help operate trains closer to that envelope. TMS can coordinate this capacity at network level.

But there is another idea behind it: Moving towards train-centric capacity means moving intelligence from trackside infrastructure to trains, data and communication systems.

This is the real transformation.

Last update: 2026-06
Reading time: 25 min

Article summary

ETCS Level 2 is a major step in railway signalling digitalisation. It brings cab signalling, radio communication, trackside-generated Movement Authorities and continuous speed supervision. But ETCS Level 2 should not be confused with Moving Block.

In conventional ETCS Level 2 implementations, the railway still generally relies on physical trackside train detection sections, such as track circuits or axle counters. The fixed-block logic remains. The difference is that movement authorities are transmitted to the cab by radio instead of being displayed mainly by lineside signals.

Hybrid Train Detection is the first step towards virtualising this fixed-block logic. It keeps a physical train detection layer, but adds a virtual layer above it. Physical trackside train detection sections can be divided into smaller Virtual Sub-Sections. For trains that can confirm their integrity and length, the trackside system can use position reports and rear-end information to release the infrastructure more finely.

Moving Block is the more ambitious step. Instead of authorising train movements mainly from fixed block boundaries or preconfigured virtual sections, a Moving Block System aims to manage train movements in a more dynamic and train-centric way. It uses train objects, movement permissions, safe front ends, safe rear ends, trackside asset states and operational data to manage safe train separation.

But this evolution has strong dependencies. To reduce train separation, the system must know where the train is, where the train ends, whether the train is complete, and what infrastructure it is running on.

This requires:

reliable train position;
train integrity;
train length;
safe rear-end information;
reliable static infrastructure data;
communication between onboard and trackside systems;
safe trackside asset control;
operational integration with TMS and Plan Execution.

This is why Advanced Safe Train Positioning, Train Integrity Monitoring, Train Length determination and the Digital Register are not secondary components. They are part of the capacity architecture.

The migration challenge is especially important for freight and variable-composition trains. Modern multiple units are easier to equip because they usually have a train bus, known consist configuration and integrated onboard systems. Legacy freight trains are different: they often consist of old wagons, mechanical couplers, pneumatic brake pipes and no continuous electronic train bus. For them, Digital Automatic Coupling and possibly electro-pneumatic braking become important enablers.

There is also an economic question. A lighter infrastructure may reduce some trackside equipment, maintenance and failure exposure. But the functions do not disappear. They move towards trains, onboard systems, data, software, certification, cybersecurity and lifecycle maintenance.

The business case cannot be assessed only from the infrastructure manager’s perspective. It must also include the investment required from railway undertakings.

Hybrid Train Detection and Moving Block are therefore not only advanced ETCS functions. They are a migration towards a different railway capacity architecture.

Content

Introduction
Why existing infrastructure capacity matters
Railway capacity starts with safe train separation
ETCS Level 2: digitalising fixed-block signalling
Hybrid Train Detection: the first layer of virtualisation
The onboard enablers: position, integrity and length
Migration: freight and variable-composition trains
Digital Register: the static data backbone
Moving Block: from virtual sections to dynamic separation
From route-centric to train-centric signalling
Why ERTMS/ATO becomes necessary
Capacity is a system property
The cost-benefit question: lighter infrastructure, more complex trains
Safety and migration
Conclusion

1. Introduction

The railway sector often describes ETCS as a train protection system. This is correct. ETCS supervises the train against its authorised movement, speed limits, braking curves and permitted operating conditions. It ensures that the train remains within its Movement Authority and intervenes if the train risks exceeding the supervised limits.

But ETCS is also part of a broader capacity story. The reason is simple: the way a signalling system authorises train movement directly influences how close two trains can safely run.

In a conventional fixed-block railway, the infrastructure is divided into sections. A train occupies a section. The following train must remain behind the protection logic associated with that section and the following safety margins. The granularity of the infrastructure, the length of the blocks, the braking distances and the operating rules strongly influence capacity.

ETCS Level 2 digitalises this architecture. It transfers signalling information to the cab, uses radio communication and allows continuous supervision. But conventional ETCS Level 2 does not automatically remove fixed blocks. In many implementations, the system still relies on track circuits or axle counters to determine whether physical sections are occupied or free.

Hybrid Train Detection goes one step further. It adds a virtual layer inside these physical train detection sections. If a train can confirm its integrity and provide reliable rear-end information, the trackside system can determine that some parts of a physical section are already clear even if the whole physical section is still reported occupied. This can reduce headways and increase the useful capacity of the line.

Moving Block goes further again. It aims to move from a fixed or virtual block logic towards a more dynamic train-centric separation logic.

But this evolution does not happen by magic. It requires trains that can report their position safely. It requires knowledge of train length and train integrity. It requires reliable static infrastructure data. It requires onboard and trackside systems to cooperate. It requires safe control of points and other trackside assets. It requires operational integration with traffic management. It requires a migration strategy for trains that are not yet able to provide the required data.

This is why Hybrid Train Detection and Moving Block should not be seen as isolated signalling upgrades. They are part of a wider architecture of railway capacity.

2. Why existing infrastructure capacity matters

Railway capacity is becoming a strategic issue. The demand for rail transport is expected to increase in many areas. Metropolitan regions need higher service frequency. Main corridors need more passenger and freight capacity. Climate policies encourage modal shift. Rail is expected to absorb part of the demand currently carried by roads and short-haul aviation.

But building new railway infrastructure is difficult. New lines require land, planning, environmental assessment, public funding, political support, construction capacity and social acceptance. They can take decades to deliver. In dense urban areas, adding new tracks can be extremely expensive. In rural or environmentally sensitive areas, new infrastructure can face strong opposition.

This does not mean that new infrastructure is unnecessary. In many cases, it remains essential. But the existing network must also be pushed closer to its useful potential.

This is where advanced signalling becomes strategic. If the railway can safely reduce the distance between trains, it can increase the capacity envelope of existing infrastructure. If it can also operate trains more precisely and more predictably, it can convert part of this theoretical capacity into usable operational capacity.

Climate change adds another dimension, but it should not be overdeveloped here. Future railway systems will need to remain usable under more frequent degraded operating conditions: heatwaves, heavy rainfall, flooding, storms, landslides, vegetation-related incidents or temporary speed restrictions. A capacity architecture designed today should therefore not only optimise nominal operation. It should also support more resilient operation.

Advanced signalling, automatic train operation and traffic management are not the whole answer. But they are part of the answer. They make it possible to use the existing infrastructure more intelligently.

3. Railway capacity starts with safe train separation

A railway line cannot be filled with trains without considering the distance required to keep them safe. Each train needs a movement authority. It needs braking distance. It needs speed supervision. It needs protection against conflicting movements. It needs assurance that the track ahead is available and safe.

The following train cannot simply move up to the rear of the preceding train. The signalling system must ensure that the following train can stop before reaching a danger point, even if the preceding train stops or if the movement authority cannot be extended. This is why train separation is central to capacity.

In a fixed-block system, the line is divided into track sections. If the section is occupied, the following train cannot be authorised into it, or can only do so under restricted conditions. The block boundary becomes the basic granularity of the capacity system.

The shorter the safe separation, the more trains can theoretically run on the same infrastructure.

But shortening safe separation requires more information. The system must know where the train is. It must know how far the train can go. It must know whether the train ahead has completely cleared the track. It must know the braking capability of the following train. It must know the gradients, speeds, route conditions and trackside asset states. It must know whether the data used to calculate the authority are reliable.

The key question is therefore not only: where is the front of the train?

It is also: where is the rear of the train?

This is a fundamental point. The front end of the preceding train tells the system where the train has reached. But the rear end tells the system what has actually been cleared behind it. If the railway wants to reduce the separation between trains, the rear end becomes essential.

Capacity starts at the rear end of the train. This is why train integrity and train length become so important for Hybrid Train Detection and Moving Block.

4. ETCS Level 2: digitalising fixed-block signalling

ETCS Level 2 is a major step in railway signalling. It introduces cab signalling, radio communication, trackside-generated Movement Authorities and continuous train supervision. The driver receives information in the cab. The train communicates with the Radio Block Centre. The system supervises speed and distance against the Movement Authority.

In full ETCS Level 2 operation, lineside signals may disappear from the driver’s perspective. But this does not mean that ETCS Level 2 has automatically left fixed-block signalling behind.

In conventional ETCS Level 2 implementations, the infrastructure is still generally divided into physical train detection sections. These sections are monitored by track circuits, axle counters or other trackside train detection systems. The interlocking receives or processes the occupation of these sections. The Radio Block Centre uses the route and track status to generate ETCS Movement Authorities.

The fixed-block logic remains. What changes is the way the information is transmitted and supervised. The authority is no longer primarily displayed by lineside signals. It is transmitted to the train by radio. The train is continuously supervised by ETCS onboard. The driver receives movement information in the cab. Eurobalises are used mainly for location referencing and correction of odometry.

ETCS Level 2 is a digital signalling system. But it is not necessarily a Moving Block system. It can remove lineside signals while still relying on fixed physical train detection sections.

This distinction is essential for the rest of the article. Hybrid Train Detection does not come after a fully dynamic ETCS Level 2. It comes after conventional Level 2 fixed-block operation. It adds a virtual layer above the physical train detection sections.

Figure 1 — From fixed blocks to Moving Block: the capacity trajectory from conventional signalling to automated operation.

5. Hybrid Train Detection: the first layer of virtualisation

Hybrid Train Detection, also known historically as Hybrid Level 3, is a pragmatic step between conventional ETCS Level 2 and a more ambitious Moving Block architecture. The principle is to keep part of the physical trackside train detection layer, while adding smaller virtual sections inside it.

In conventional ETCS Level 2, the end of authority is generally constrained by fixed physical sections. A train detection section is free or occupied. If it is occupied, the system must normally treat the whole section as unavailable for the following train.

Hybrid Train Detection changes the granularity. A physical train detection section can be divided into Virtual Sub-Sections. These virtual sections are not detected by trackside equipment. They are calculated by the trackside system using the position reports sent by the train, the train length and the confirmation that the train is still complete.

If the train can confirm its integrity, the system can use the reported rear-end position to determine which virtual sections have been cleared. The following train can then potentially receive a Movement Authority closer to the rear of the preceding train.

This can reduce the headway. It can also reduce the need for dense trackside train detection. Instead of installing more axle counters or track circuits to create shorter physical blocks, the system can create shorter virtual sections in the configuration of the trackside system.

This is the beauty of Hybrid Train Detection. It can increase the granularity of train separation without necessarily increasing the number of trackside detection devices.

But the word “hybrid” is important. Hybrid Train Detection does not simply remove trackside train detection. It still keeps physical detection for important purposes:

to support trains that cannot confirm integrity;
to handle degraded situations;
to detect non-connected trains;
to support migration;
to maintain safety when onboard information is unavailable or unreliable.

This makes HTD a migration architecture. It does not require every train to be fully moving-block capable from day one. But it creates capacity benefits for trains that can provide the required onboard information. It is therefore both a capacity step and a migration compromise.

Figure 2 — Hybrid Train Detection principle: physical detection remains, but virtual sub-sections allow track to be released more finely.

6. The onboard enablers: position, integrity and length

Reducing train separation requires the train to become a reliable source of information. This is a major shift.

In conventional fixed-block signalling, the infrastructure detects the presence of the train. Track circuits or axle counters determine whether a section is occupied. The train itself does not need to tell the trackside system exactly where its rear end is for the signalling system to release a block.

In Hybrid Train Detection and Moving Block, this changes. The system needs more information from the train.

It needs:

safe train position;
train speed and possibly acceleration;
train front-end location;
train rear-end location;
train length;
train integrity;
confidence in the reported information.

This is where several onboard enablers become essential.

Advanced Safe Train Positioning

Advanced Safe Train Positioning, or ASTP, is the onboard positioning layer. Its purpose is to provide more accurate and safe localisation information than today’s conventional ETCS odometry principles alone.

ASTP can combine several sources of information: odometry, balise information, GNSS or satellite-based positioning, inertial sensors, map data, augmentation data and other supporting inputs depending on the architecture.

The key point for this article is not the detailed sensor fusion. The key point is this: If trackside train detection is reduced, the train must become more capable of safely locating itself.

The infrastructure no longer carries the whole burden of localisation and occupation detection. The onboard system becomes part of the capacity architecture.

ASTP is therefore not only a positioning technology. It is an enabler for a more train-centric railway.

Train integrity

Train integrity answers a simple but safety-critical question: Is the train still complete?

If the train has separated unintentionally, the front part of the train may continue to report its position, but the rear part may remain on the track. In such a situation, the infrastructure behind the front part cannot be considered safely cleared.

For conventional passenger multiple units, train integrity may be easier to determine because the train is a fixed or semi-fixed composition with integrated onboard systems.

For freight trains, it is much more difficult. This is why train integrity is one of the central challenges of Hybrid Train Detection and Moving Block.

Train length

Train length answers another essential question: where is the rear end of the train?

If the system knows the front end of the train but not its length, it cannot determine the rear end precisely. And without a reliable rear end, it cannot safely release the infrastructure behind the train at a finer granularity.

In ETCS principles, train integrity information reported to the RBC can include the integrity status and confirmed train length. This allows the trackside to use confirmed rear-end information.

This is the capacity value of train length. Train length is not only a train data parameter. It becomes an operational input for the release of track behind the train.

The capacity formula

To reduce safe train separation, the system must know four things:

where the train is;
where the train ends;
whether the train is complete;
what infrastructure it is running on.

This is the foundation of train-centric capacity.

Figure 3 — Onboard enablers of reduced train separation: position, integrity and rear-end information become capacity inputs.

7. Migration: freight and variable-composition trains

Hybrid Train Detection and Moving Block are easier to imagine with modern passenger rolling stock.

Modern multiple units usually have integrated onboard systems, a train bus, a known configuration, TCMS, onboard diagnostics and a more controlled train composition. Equipping such trains with train integrity, train length and advanced positioning functions is still a technical and economic challenge, but the architecture is more favourable.

Freight is different.

A conventional freight train in Europe is often composed of wagons of different ages, different owners and different technical standards. Many wagons are old. They are mechanically coupled. The only continuous link along the train is often the pneumatic brake pipe. There is no electronic train bus. There is no native onboard system that can automatically determine the integrity and length of the complete train.

This is a major migration issue. If a future infrastructure were operated exclusively in a train-centric Moving Block mode, requiring all trains to provide reliable position, integrity and length, many existing freight trains would not be able to operate on it without major retrofit.

The same challenge can apply to locomotive-hauled passenger trains, push-pull trains, variable-composition trains, engineering trains or special trains. Everything that is not a modern fixed-consist multiple unit can become more difficult.

This is where the Digital Automatic Coupler becomes important. DAC is often presented as an operational and productivity enabler for freight: automated coupling, better handling of train composition, improved efficiency in yards, digital continuity along the train.

But in the context of Hybrid Train Detection and Moving Block, DAC can become something more. It can become an enabler for train integrity and train length.

If freight wagons are connected by a digital coupling architecture, it becomes possible to exchange information along the train and to build a more reliable view of the train composition. This can support onboard train integrity functions and train length determination.

Electro-pneumatic braking is another related topic. A long freight train using only conventional pneumatic brake propagation does not react like a modern multiple unit. The brake command propagates progressively along the brake pipe. On very long trains, the delay can be significant. A more dynamic capacity architecture must also consider braking performance, brake build-up time and predictability. Electro-pneumatic braking could support faster and more homogeneous brake command propagation along the train.

But this is another migration challenge. It requires investment, retrofit, certification, maintenance and operational change.

The conclusion is important: The hardest part of train-centric capacity may not be the passenger train. It may be freight.

A capacity architecture that ignores freight migration could unintentionally create a system that works technically, but excludes part of the railway market from the most capacitive infrastructure. This would be a strategic mistake.

Figure 4 — Freight migration challenge: train-centric capacity is easier for modern multiple units than for legacy freight trains.

8. Digital Register: the static data backbone

A dynamic signalling system still depends on static data. Moving Block is dynamic. Train separation becomes more dependent on train position, train length, safe rear end, movement permissions and real-time operational state. But none of this can be safe or reliable if the infrastructure data are wrong.

The system must know the topology of the network. It must know the location of track sections, points, gradients, speed profiles, trackside assets, restrictions and operational boundaries. It must know how the infrastructure is connected. It must know what data version is valid. It must ensure that the same infrastructure description is used consistently by the systems that depend on it. This is the role of the Digital Register.

In FP2-R2DATO, the Digital Register is introduced as a centralised infrastructure data management system. It provides reliable static infrastructure data to several systems: TMS, Plan Execution, Moving Block System, ASTP, Perception, Automatic Processing Module, Automatic Driving Module and others.

For Hybrid Train Detection and Moving Block, this is essential. If ASTP uses map data for localisation, the map must be reliable. If the Moving Block System uses domain data to manage train movements, the data must be consistent. If TMS and Plan Execution coordinate the operation, they must refer to the same infrastructure reality. If ATO follows a profile, it must be based on reliable track and route information.

The phrase is simple: Moving Block is dynamic, but it depends on static data being correct.

This is why the Digital Register is not a documentation tool. It is part of the safety and capacity architecture.

9. Moving Block: from virtual sections to dynamic separation

Hybrid Train Detection virtualises fixed-block logic. Moving Block aims to go further.

In a Moving Block System, the railway is no longer primarily managed as a sequence of fixed sections that are free or occupied. The system manages train objects, movement permissions, safe front ends, safe rear ends, trackside assets, usage restrictions and operational state.

The objective is to authorise train movement more dynamically. Instead of granting movement up to a fixed section boundary, the system can calculate safe movement based on the position and protected envelope of trains, the state of infrastructure, the braking capability of trains and the operational plan.

This is a deeper architectural shift. The Moving Block System must know not only that a section is occupied, but where the train is, where it ends, what part of the track is protected for it, what assets are aligned, what restrictions apply, and what other trains may conflict with it.

This is why FP2-R2DATO describes Moving Block as train-centric. The train becomes the central moving object around which the safety logic is built.

This does not mean that trackside assets disappear. Points still exist. Level crossings still exist. Movable bridges still exist. Operational restrictions still exist. Trackside assets still need to be commanded, supervised and protected.

But the logic is no longer simply a traditional block system mainly structured around signals. It becomes a control architecture centred on safe train movement.

Moving Block is therefore not just “shorter blocks”. It is a different way to organise the signalling system.

10. From route-centric to train-centric signalling

There is also an important architectural difference between conventional ETCS Level 2, Hybrid Train Detection and Moving Block.

In a conventional ETCS Level 2 deployment, the interlocking remains the core route-setting system. It commands and locks routes. It controls points and other trackside assets. It receives or processes track occupation information from train detection systems. It ensures that conflicting routes cannot be set and that the infrastructure is in the correct state for a train movement.

The RBC is then connected to this signalling environment. It receives route, signal and track status information, and converts the safe movement information into ETCS Movement Authorities that can be transmitted to the train.

In this sense, the RBC is often an ETCS layer placed on top of an existing national signalling and interlocking architecture.

Hybrid Train Detection does not fundamentally break this logic. It adds a virtualisation layer. The physical train detection sections still exist. The legacy route-setting principles can remain. But the system can use Virtual Sub-Sections, train position reports, train integrity and train length to release parts of the infrastructure more finely.

Moving Block is more disruptive. In the train-centric FP2-R2DATO approach, the conventional separation between RBC and interlocking is no longer imposed. The Moving Block System manages train objects, movement permissions, trackside assets and operational state within its Area of Control.

This does not mean that all interlocking-like safety functions disappear. They remain necessary. Points must still be safely controlled. Conflicting paths must still be prevented. Trackside assets must still be supervised. The infrastructure must still be protected.

But these functions can be reorganised in a broader Moving Block System architecture. The movement is therefore from route-centric signalling towards train-centric control.

This is not a small change. It affects engineering, safety cases, interfaces, operational procedures, maintenance, migration and national signalling architectures.

11. Why ERTMS/ATO becomes necessary

Hybrid Train Detection and Moving Block can increase the capacity envelope of a railway line. But a capacity envelope is not the same thing as usable capacity.

A signalling system may allow trains to run closer to each other. But in real operation, trains are driven by humans with natural variability. Drivers do not all brake in the same way. They do not all accelerate in the same way. They do not all react at the same moment. They do not always follow the most energy-efficient or capacity-efficient profile.

Even if all drivers are competent and safe, variation exists.

When a line is operated far from its capacity limit, this variation may be acceptable. When a line is operated closer to its capacity envelope, variation becomes more critical. This is where ERTMS/ATO becomes important.

ATO does not create the safety distance. ETCS does.

ATO does not replace train protection. ETCS remains the safety layer.

But ATO can help operate trains more precisely within the envelope defined by ETCS, HTD or Moving Block. It can follow the Journey Profile. It can reduce variability between driving behaviours. It can respect speed profiles more consistently. It can optimise traction and braking. It can support energy-efficient driving. It can make train movement more predictable for the TMS.

This is the right articulation: HTD and Moving Block can create a higher signalling capacity envelope. ERTMS/ATO helps operate trains close to that envelope.

Without ATO, the capacity gain created by HTD or Moving Block may remain partly theoretical. With ATO, the railway has a better chance to convert theoretical capacity into stable operational capacity.

12. Capacity is a system property

It would be wrong to suggest that Hybrid Train Detection or Moving Block alone can solve railway capacity. They address one of the key constraints: train separation. But capacity also depends on many other factors.

It depends on rolling stock performance. A train with poor acceleration or braking performance will not exploit the same capacity envelope as a high-performance multiple unit.
It depends on station dwell time. A line can have excellent signalling, but if trains stay too long in stations, capacity remains constrained.
It depends on junctions. A flat junction can create conflicts that no moving block logic can remove without changing the infrastructure or the timetable.
It depends on terminal capacity. If trains cannot turn back fast enough, the line capacity cannot be fully used.
It depends on timetable design. A heterogeneous timetable mixing fast, slow, passenger and freight trains will not use capacity in the same way as a homogeneous high-frequency service.
It depends on TMS. If conflicts are not detected, managed and resolved dynamically, capacity becomes fragile.
It depends on Plan Execution. The operational plan must be translated into safe executable requests.
It depends on data. If the Digital Register is wrong, the system is wrong.
It depends on onboard systems. If the train cannot provide position, integrity and length, the system must become conservative.
It depends on degraded modes. If the system loses communication or integrity information too often, the capacity benefit disappears.

This is why the correct framing is: Hybrid Train Detection and Moving Block are not capacity solutions by themselves. They are capacity enablers inside a wider system architecture.

Capacity is not only a signalling value. It is the result of the complete railway system.

13. The cost-benefit question: lighter infrastructure, more complex trains

Hybrid Train Detection and Moving Block are often associated with the idea of reducing trackside equipment. This can be true. If physical train detection sections can be longer or less dense, if virtual sections replace some physical detection devices, and if the system relies more on onboard position and integrity information, the infrastructure can become lighter.

This may reduce some infrastructure CAPEX. It may also reduce some OPEX: fewer track circuits, fewer axle counters, fewer physical assets exposed to weather, fewer assets to inspect, maintain and renew. It may also improve availability if fewer trackside assets fail.

But this is only half of the economic story. The functions do not disappear. They move. The train becomes more complex.

It may need ASTP. It may need train integrity monitoring. It may need train length determination. Freight may need DAC. Some operations may need electro-pneumatic braking. Onboard software becomes more important. Cybersecurity becomes more important. Data management becomes more important. Certification becomes more complex. Maintenance shifts from trackside assets to onboard systems and digital architecture.

A lighter infrastructure is not a cost-free architecture. It is a cost-shift architecture. This creates an important sector question. Who pays?

The infrastructure manager may benefit from fewer trackside assets and higher line capacity. The railway undertaking may have to invest in onboard equipment. Freight operators may have to retrofit low-margin assets. Wagon keepers may face new requirements. Public authorities may benefit from more capacity and modal shift. Passengers and shippers may benefit from better service.

But the investment burden may not naturally fall where the benefits appear. This is a serious migration issue.

The business case of HTD and Moving Block cannot be assessed only from the infrastructure manager’s perspective. It must include the rolling stock investment required from railway undertakings. If the infrastructure becomes lighter but the train becomes more complex, the sector must decide how the value created by capacity is shared.

This question is still not sufficiently mature. It must become part of the deployment discussion. Otherwise, the technology may be technically sound but economically difficult to deploy.

Figure 5 — Capacity cost-shift architecture: lighter infrastructure can mean more onboard functions, data and lifecycle cost.

14. Safety and migration

Reducing train separation is not only a capacity decision. It is a safety architecture decision.

The more dynamic the separation becomes, the more important the correctness of the information becomes.

If train position is wrong, the system may release infrastructure incorrectly.
If train length is wrong, the rear end may be assumed to be somewhere it is not.
If train integrity is lost, the system must become conservative.
If communication is degraded, the system must know how to fail safely.
If Digital Register data are wrong, the system may calculate movement on a wrong infrastructure representation.
If a non-connected train enters the area, the system must not assume that virtual sections are clear.
If a freight train cannot confirm integrity, the system must not treat it as a train that can release virtual sections.

This is why migration matters. A railway cannot switch overnight from conventional fixed-block operation to full train-centric Moving Block.

There will be mixed traffic. There will be equipped and non-equipped trains. There will be modern passenger multiple units and legacy freight trains. There will be national systems, ETCS Level 2, Hybrid Train Detection and possibly Moving Block environments. There will be degraded modes. There will be transitions between areas.

The migration architecture must therefore answer several questions:

Can non-integrity-confirmed trains run on an HTD line?
How much capacity is lost when they do?
How are virtual sections managed when integrity is lost?
How are non-connected trains detected or protected against?
How is freight accommodated?
What happens during communication loss?
How are transitions between ETCS Level 2, HTD and Moving Block handled?
How are route-centric and train-centric systems connected?
How are operational rules harmonised?
How are responsibilities shared between IM, RU and onboard systems?

The safety analysis of Moving Block cannot be limited to individual components. It must analyse interactions. This is especially important because the system becomes more distributed. Safety depends on trackside, onboard, communication, data, operations and human procedures.

The more the railway becomes digital, connected and train-centric, the more safety becomes architectural.

15. Conclusion

ETCS Level 2 is a major step in railway signalling digitalisation. It brings cab signalling, radio communication and continuous train supervision. But conventional ETCS Level 2 remains largely compatible with the fixed-block logic inherited from conventional signalling. The movement authority is digital and supervised in the cab, but train separation can still rely on physical trackside train detection sections.

Hybrid Train Detection is the first step towards virtualising this logic. It keeps physical train detection, but adds Virtual Sub-Sections. It uses train position reports, train integrity and train length to release track more finely behind trains that can confirm they are complete.

Moving Block is the more ambitious step. It moves towards a train-centric architecture where movement permissions, train objects, safe front ends, safe rear ends, trackside assets and operational state are managed dynamically.

But none of this can work without new dependencies.

The train must know where it is.
The system must know where the train ends.
The train must be able to confirm its integrity.
The infrastructure data must be reliable.
The onboard and trackside systems must cooperate.
The TMS must coordinate the operation.
The ATO must help operate close to the resulting capacity envelope.

This is why Hybrid Train Detection and Moving Block are not only signalling topics. They are system architecture topics.

They also raise difficult migration questions. Freight trains, variable-composition trains and legacy rolling stock cannot be ignored. A railway capacity architecture that works only for modern multiple units would not be sufficient for the European railway system. DAC, train integrity, train length, electro-pneumatic braking and onboard digitalisation become part of the freight migration challenge.

There is also an economic question. Reducing trackside equipment may make infrastructure lighter and potentially cheaper to maintain. But the cost does not disappear. Part of it moves to rolling stock, onboard systems, software, data, cybersecurity and certification.

The sector must therefore answer not only the technical question: Can we reduce safe train separation?

But also the migration and business question: Who invests, who benefits, and how do we ensure that the whole railway system can move towards this architecture?

The direction is clear. Existing infrastructure must be used better. ETCS must evolve from train protection towards capacity architecture. Hybrid Train Detection provides a pragmatic virtualisation step. Moving Block provides the more ambitious train-centric target. ERTMS/ATO helps operate trains close to the new envelope. TMS coordinates capacity at network level.

This is the bridge between ETCS and ERTMS/ATO. ETCS protects the train. Hybrid Train Detection and Moving Block increase the capacity envelope. ERTMS/ATO helps make this capacity usable.

And together, they define one of the most important transitions in European railway automation.

Documentation

Voie Libre articles

European and R&D documentation

ERA / UNISIG / EEIG ERTMS Users Group — SUBSET-026-2, System Requirements Specification, Chapter 2, Basic System Description, Issue 4.0.0
ERA / UNISIG / EEIG ERTMS Users Group — SUBSET-026-3, System Requirements Specification, Chapter 3, Principles, Issue 4.0.0
FP2-R2DATO — D15.1, HL3 System Specification with identification of use cases and related engineering rules
FP2-R2DATO — D13.1, Moving Block Specifications applying a train-centric approach, Introduction
FP2-R2DATO — D13.1, Part 1, System Definition
FP2-R2DATO — D13.1, Part 2, System Specification
FP2-R2DATO — D13.1, Part 3, Safety Analysis
FP2-R2DATO — D19.1, Train Integrity and Train Length Architecture & Functional Requirements
FP2-R2DATO — D21.1, Operational requirements and system capabilities of an ASTP system
FP2-R2DATO — D21.2, System requirements of ASTP system
FP2-R2DATO — D27.2, Specification of Digital Register Implementation(s) required in R2DATO
FP1-MOTIONAL — D8.3, Developed Simulation Methods and Models for Capacity Evaluation of ETCS and C-DAS/ATO