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.