Principle of moving block

Traditional signalling systems are based on so called fixed blocks: the railway is divided into

sections of track, which are separated by signals. A train is not allowed to enter a given track section (=block) before the preceding train has cleared it. This system has a number of disadvantages, one being its lack of flexibility: the block size is the same for all trains regardless of their speed and braking performance. Thus the big safety distances required by fast trains are imposed on slower trains as well. Obviously this reduces track capacity.

A moving block system (often called CBTC = Communications Based Train Control) does not require traditional fixed-block track circuits for determining train position. Instead, it relies on continuous two-way digital communication between each controlled train and a wayside control centre.

On a moving block equipped railway, the line is usually divided into areas or regions, each area under the control of a computer and each with its own radio transmission system. Each train transmits its identity, location, direction and speed to the area computer which makes the necessary calculations for safe train separation and transmits this to the following train. The radio link between each train and the area computer is continuous so the computer knows the location of all the trains in its area all the time. It transmits to each train the location of the train in front and gives it a braking curve to enable it to stop before it reaches that train. In effect, it is a dynamic distance-to-go system. As long as each train is travelling at the same speed as the one in front and they all have the same braking capabilities, they can, in theory, run as close together as a few metres (e.g. about 50 metres at 50 km/h). This, of course, would contradict the railways safety policies. Instead, one safety feature of fixed block signalling is usually retained - the requirement for a full speed braking distance between trains. This ensures that, if the radio link is lost, the latest data retained on board the following train will cause it to stop before it reaches the preceding train.

What distinguishes moving block from fixed block is that it makes the block locations and lengths consistent with train location and speed, i.e. making them movable rather than fixed.

Potential application contexts for moving block:

ETCS level 3:

The European Train Control System (ETCS) containing the interoperability specifications of future train operation in Europe will be introduced in three steps (ETCS Level 1 – 3). The introduction of moving block train control will not be possible before level 3 is reached. This level is characterised by the following features:

• Bidirectional information transmission between train and the Radio Block Center (RBC) with the GSM-R standard.
• Train positioning by EUROBALISES (beacons) installed every 1000 m between the rails. On the sections between two balises positioning is realised by radar and odometers allowing very high levels of accuracy.
• Continuous and safe speed control
• On-board train integrity check (presently done by track-side axle-counters)
• virtual block control by the Radio Block Centers (=moving block)
• Only trains equipped for level 3 are allowed on level 3 tracks.

Radio control for regional lines

The principles of ETCS and GSM-R developed for the European high speed, cross-border traffic may also be used in other contexts. The German DB AG currently tests the introduction of a radio control based on the ETCS elements EURORADIO (using GSM-R), EUROBALISE and EUROCAB on some of its regional lines (Funkfahrbetrieb (FFB)). The French Reseau Ferroviaire Francaise (RFF) will realise a similar pilot project. Radio control on regional lines does not necessarily imply moving block principles but allows them as a option.

Seltrac by Alcaltel

One Seltrac system marketed by Alcatel claims to be the first moving block system is that. It is used in Canada and on the Docklands Light Railway in London. Even though it has the key components of moving block systems, data transmission is not realised by a radio link but by track-mounted induction loops installed between the rails and crossing every 25 metres allowing trains to verify their position. Data is passed between the vehicle on-board computer (VOBC) and the vehicle control centre (VCC) through the loops. The VCC controls the speed of Train 2 by checking the position of Train 1 and calculating its safe braking curve. The Seltrac system requires no driver, as it is fully automatic. In case of a system failure axle counters allow for train positioning. The main drawback is the need for continuous cables to be installed within the tracks, which are expensive to install and vulnerable to damage during track maintenance.

Evaluation focus

Different realisations of the moving block concept are conceivable. The following evaluation concentrates on the versions envisioned by ETCS level 3 (including its limited introduction on regional lines).

The operational principle of moving block and the introduction of radio control are closely linked and therefore cannot be evaluated separately. However one must keep in mind that radio control is at the basis of moving block but does not necessarily imply it.

Whereas radio controlled train operation has reached the stage of pilot projects, so far no realisation of moving block based on radio transmission is known.

• Seltrac by Alcatel, an early and reduced version of moving block, has reached the application stage.
• The ETCS level 3 is confronted with some problems of technological and operational nature still to be resolved. After Railtrack retreated from equipping its West Coast Main Line with ETCS Level 3, there are presently no plans for introducing this level on any line in short term perspective. The new German high speed line Cologne – Frankfurt, opened in late 2002 for 300 km/h, could be appropriate to test ETCS service, since it will be limited to modern high speed trains. There is a number of European lines where all ETCS levels will be tested in the next years (on of them being the DB line Ludwigsfelde - Jüterbog).
• Pilot projects for an ETCS-analogue radio control on regional lines are under way in Germany and France. While theoretically allowing moving block as an option, these lines will be operated in the near future by a radio controlled fixed-block.

• Improvements in operations and in operating capacity (motivation for moving block)
• Reduction of fixed infrastructure costs, since expensive wayside equipment is replaced by on-board devices (motivation for communication based train control not necessarily implies moving block)

Increased capacity

• The development should lead to more regular traffic flows with fewer delays. Exploitation of Moving Block capabilities will permit railway administrations and operators to increase line capacity, utilise the network infrastructure more efficiently, and improve service.
• The improvement in capacity will provide opportunities for independent operators to use the common infrastructures.

Transition costs

The transition from track-side signalling to GSM-R based “virtual” signalling is confronted by a number of specific problems of economic and organisational nature:

• Longevity of systems: Typical depreciation periods of train control systems are about 20 years. It is therefore not economical to abandon the system earlier. This issue is particularly relevant if national authorities are involved in financing infrastructure measures (as is the case in Germany).
• Cost distribution: Even though the technical transition from track-side signalling to GSM-R based “virtual” signalling is expected to prove economical from an overall perspective, the track-side savings have to be paid by additional investments for vehicle equipment. Consequently acceptance problems arise if infrastructure and vehicles are managed by different companies. This issue is virulent even within the DB AG, where the infrastructure operator DB Netz profits from the transition whereas DB Cargo and DB Reise are confronted with additional costs.
• Apart from these intrinsic problems of the technological transition, the international character of the ETCS process brings about lengthy decision making and standardisation processes.

Technological shortcomings

ETCS level 3 poses a number of technical challenges to be resolved:

• To the present day there exists no satisfying solution for the on-board train integrity check of freight trains. The US principle of „end of train telemetry“ with a radio device on the last coach is rejected by European operators due to difficult logistics.
• There remain uncertainties concerning the ways for securing switch sections. In a fixed block system this is done by the signalling stations, in a ETCS 3 system the train distance is to be controlled by the Radio block centres. Their operating logics is to be clarified as far as switch areas are concerned.
• Security standards pose very high requirements on the preciseness and infallibility of train positioning. Although this challenge seems to be resolved in a satisfying manner by a combination of EUROBALISES, radar and odometers, the system still awaits the official operating permission by the railway institutions (respective tests for operation permits by the German EBA (Eisenbahnbundesamt) will be finished by 2003).

Uncertainties about effect on traffic fluidity

The performance benefits of moving block as compared to fixed block operation is questioned by some authors. They hold that contrary to what is generally believed the difference in track performance of fixed block with shortened block lengths as compared to moving block operation is negligible because the performance of a given track today is much more influenced by timetable structure than by distance control.

• As far as capacity and smooth traffic flow are concerned, moving block is most effective if the train speeds on a given track are homogeneous. A principal success factor for moving block is therefore the strategy of network separation.
• Energy efficiency is not a key driver for the introduction of moving block. Nevertheless, more reliable information on the efficiency potential of moving vs. fixed block train control is needed and could supply additional arguments for radio-controlled train operation.

Moving block systems are expected to lead to a smoother traffic flow and to a considerable reduction of train stops (or decelerations) along the track. The reason is very simple. In a fixed block system trains often have to stop because the track section ahead is not cleared yet, although the braking distance from the preceding train would allow to continue.

This effect is widely accepted. It is however very difficult to quantify. Quantification would require knowledge of

• the average number of stops along the track imposed by fixed block signalling
• the share of those stops being eliminated by moving block
• the average energy consumed per stop

Some experts warn, not to overestimate the effect of moving block on traffic fluidity and suggest to consider other measures as well, e.g. Optimisation of train operation by control center and demixing.