ATP Beacons and Moving Block
This page has further information on Automatic Train Protection (ATP) with descriptions of beacons and moving block systems. More articles on this site about signalling are listed in the Signalling Index
ATP Code Transmission
We have seen in the previous articles that the ATP signalling codes contained in the track circuits are transmitted to the train. They are detected by pick-up antennae (usually two) mounted on the leading end of the train under the driving cab. This data is passed to an on-board decoding and safety processor. The permitted speed is checked against the actual speed and, if the permitted speed is exceeded, a brake application is initiated. In the more modern systems, distance-to-go data will be transmitted to the train as well. The data is also sent to a display in the cab which allows the driver of a manually driven train to respond and drive the train within the permitted speed range.
At the trackside, the signal aspects of the sections ahead are monitored and passed to the code generator for each block. The code generator sends the appropriate codes to the track circuit. The code is detected by the antennae on the train and passed to the on-board computer. As we have seen, the computer will check the actual speed of the train with the speed required by the code and will cause a brake application if the train speed is too high.
In the examples so far, the ATP data from the track to the train is transmitted by using coded track circuits passing through the running rails. It is known as the "continuous" transmission system because data is passing to the train all the time. However, it does have its limitations. There are transmission losses over longer blocks and this reduces the effective length of a track circuit to about 350 metres. The equipment is also expensive and vulnerable to bad weather, electronic interference, damage, vandalism and theft. To overcome some of these drawbacks, a solution using intermittent transmission of data has been introduced. It uses electronic beacons placed at intervals along the track.
In the best known system, originally developed by Ericsson in Sweden and formerly marketed by Adtranz (now Bombardier), there are usually two beacons, a location beacon to tell the train where it is and a signalling beacon to give the status of the sections ahead. The beacons are sometimes referred to as "balises" after the French. Data processing and the other ATP functions are similar to the continuous transmission system.
Operation With Beacons
The beacon system operates as shown in the simplified diagrams below. In the diagram (left), the beacon for red Signal A2 is located before Signal A1 to give the approaching train (2) room to stop. Train 2 will get its stopping command here so that it stops before it reaches the beacon for signal A3.
In the diagram on the left, the train has stopped in front of Signal A2 and will wait until Train 2 clears Block A2 and the signal changes to green. In reality, it will not move even then, since it requires the driver to reset the system to allow the train to be restarted. For this reason, this type of ATP is normally used on manually driven systems.
A disadvantage of the beacon system is that once a train has received a message indicating a reduced speed or stop, it will retain that message until it has passed another beacon or has stopped. This means that if the block ahead is cleared before Train 2 reaches its stopping point and the signal changes to green, the train will still have the stop message and will stop, even though it doesn't have to. Why, might you ask, can't the driver cancel the stop message like he does when the train has stopped and the signal changes to green? If he could cancel the stop message while the train was moving, the system would be no better than the AWS with its cancel button. ATP is "vital" or "fail-safe" and must not allow human intervention to reduce its effectiveness.
To avoid the situation of an unnecessary stop, an intermediate beacon is provided. This updates the train as it approaches the stopping point and will revoke the stop command if the signal has cleared. More than one intermediate beacon can be provided if necessary.
Moving Block - The Theory
As signalling technology has developed, there have been many refinements to the block system but, in recent years, the emphasis has been on attempts to get rid of fixed blocks altogether. Getting rid of fixed blocks has the advantage that you can vary the distances between trains according to their actual speed and according their speeds in relation to each other. It’s rather like applying the freeway rules for speed separation - you don’t need to be a full speed braking distance from the car in front because he won’t stop dead. If you are moving at the same speed as he is, you could, in theory, travel immediately behind him and, when he brakes, you do. If you allow a few metres for reaction time to his brake lights and variations in braking performance, it works well. Although it only needs a few spectacular collisions on the freeways to disprove the theory for road traffic, in the more regulated world of the railway, although it could not be applied without a full safe braking distance between trains, it has possibilities.
In the diagram (left), 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. Just allow some room for reaction time and small errors and trains could run as close together as 50 metres at 50 km/h. Well, that’s OK in theory but, in practice, it’s a different matter and, as yet, no one has taken moving block design this far and they are unlikely to do so in the near future. The recent ICE high speed accident in Germany where a train derailed, struck a bridge and stopped very quickly, effectively negates the safety value of the theoretical moving block system described above. This means that it is essential to maintain a safe braking distance between trains at all times.
What is worth doing, is making the the block locations and lengths consistent with train location and speed, i.e. making them movable rather than fixed. This flexibility requires radio transmission, sometimes called Communications Based Train Control (CBTC) or Transmission Based Signalling (TBS) rather than track circuit transmission, to detect the location, speed and direction of trains and to tell trains their permitted operating speed.
Moving Block and Radio Transmission
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 as shown here (left).
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. This is Communications Based Train Control (CBTC).
One fixed block feature has been 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. The freeway style vision of two trains moving at 50 km/h with 50 metres between them is a step too far into virtual reality for most operators.
Moving Block - Location Updates
As we have seen, trains in a moving block system report their position continuously to the area computer by means of the train to wayside radio. Each train also confirms its own position on the ground from beacons, located at intervals along the track, which recalibrate the train’s position compared with the on-board, computerised line map.
Transferring a train from one area to another is also carried out by using the radio links and, additionally by a link between the two adjacent area computers. The areas overlap each other so, when a train first reaches the boundary of a new area, the computer of the first area contacts the computer of the second area and alerts it to listen for the new train’s signal. It also tells the train to change its radio codes to match the new area. When the new area picks up the ID of the train it acknowledges the handover from the first area and the transfer is complete.
Another version of the moving block system has the location computers on the trains. Each train knows where it is in relation to all the other trains and sets its safe speeds using this data. It has the advantage that there is less wayside equipment required than with the off-train system but the amount of transmissions is much greater.
An Early Moving Block System
One system which claims the distinction of being the first moving block system is that marketed under the name Seltrac by Alcatel. It is used in Canada and on the Docklands Light Railway in London. It has the ingredients of moving transmission of data, but the transmission medium is the track-mounted induction loops which are laid between the rails and which cross every 25 metres to allow 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. More detail on this system is here
The Seltrac system requires no driver, as it is fully automatic. In case of a system failure where a train has to be manually driven, it has axle counters¹ to verify the position of a train not under the control of the loops. Perhaps its biggest drawback is the need for continuous cables to be laid within the tracks, expensive to install and open to damage during track maintenance.
The principle difference between this system and the more modern ones being marketed today is that Seltrac uses electro-magnetic transmission of data requiring track cables, whereas radio based systems only require aerials. Seltrac is upgrading their design to use radio based transmission.
Moving Block - Why Do We Need It?
Railway signalling has traditionally required a large amount of expensive hardware to be distributed all along a route which is exposed to variable climatic conditions, wear, vandalism, theft and heavy usage. Because of the widely spaced distribution, maintenance is expensive and often restricted to times when trains are not running. Failures are difficult to locate and difficult to reach. On metros, access is further restricted where there are tunnels and elevated sections. For these reasons, railway operators have been trying to reduce the wayside signalling equipment and so reduce maintenance costs. Reduced wayside equipment can also lead to reduced installation costs. Moving block requires less wayside equipment than fixed block systems.
There is another goal much sought after by operators - greater capacity. A norm for most metro lines is 30 trains per hour (tph) or a two-minute headway. It is debatable whether much improvement on this is possible for a high capacity system, since the major losses of line capacity occur because of station stops and terminal operations. Heavily used metro lines, like those in Hong Kong, trying for a greater capacity than 30 trains per hour, will struggle to keep dwell times below 40-50 seconds at peak times. This will push the headway to two minutes or longer, regardless of the signalling system used. Similar problems exist at terminals where crossover clearance times are critical. Moving block signalling cannot provide much improvement. Shorter headways can, however, be achieved on systems where trains are shorter, speeds lower and the passenger levels smaller. In some places a 95 second headway can be achieved on systems like Docklands and certain sections of the Paris Metro.
Also, for underground lines, modern ventilation and smoke control systems will require train separation of 2-300 metres to allow air circulation at critical times. If moving block signalling allows 50 metre separation, some very expensive additional ventilation arrangements might be necessary. This may reduce the benefits of moving block.
The real prize which could be won by an operator using moving block is reduced wayside equipment and reduced maintenance costs. Better reliability and quicker fault location is also possible with moving block technology. If radio based transmission is included, an all-round improvement can be achieved.
One other factor to be noted is that many operators specifying moving block technology also ask for fixed block track circuits to serve as a back up and for broken rail detection. Track circuits are also still required for junctions. One might ask, if such equipment is to be installed anyway, why add the expense of radio-based transmission?
1. Axle counters are sometimes used as a way of verifying that a train has completely passed through a block instead of a track circuit. The number of axles on the train are counted as the train enters the block and counted again as it exits.
More articles on signalling:
The Development and Principles of UK Signalling - Metro Signalling and ATP - ATO - Route Signalling - ATC - UK Warning & Protection Systems - UK Signalling - What the driver sees - Single Line Signalling - US Signalling - Docklands Light Railway Signalling