Deadlock (railroad)

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A deadlock ( English for "hopeless situation") is a situation in the railroad in which trains block each other so that regular train operations are no longer possible.

meaning

Example of a deadlock on a single-track route between two train stations with two tracks each

The simplest case of a deadlock is a single-track line on which two trains face each other. As long as each of the trains is in its own train heading section , there is no risk of a collision in this situation, but this hindrance is prevented by the so-called counter -run protection to secure train journeys . A deadlock that is also possible with train protection is when, during single-track train operation, a station with two tracks is occupied by two trains traveling in the same direction (e.g. to overtake ), but at the same time a train is approaching on the single-track section. Since all tracks in the station are occupied, no train crossings are possible and the trains block each other.

Deadlock in track switching operation : None of the trains in the direction of travel have any free trajectory sections marked in red .

A deadlock can also occur on double-track lines that are used in track-changing mode, if two trains are approaching each other on both tracks and it is no longer possible to leave the opposite track by changing track . In larger train stations , the condition for a deadlock is that the routes are blocked by trains that want to enter, but can no longer enter due to overcrowded station tracks. On the other hand, double-track lines, where only the regular track is used in each direction, offer great security .

The expense of eliminating such a situation by shunting is very high in railway operations. However, it is impossible to build the rail infrastructure of a complex route network in such a way that deadlocks are fundamentally excluded. Firstly, before a new timetable is introduced , which has been coordinated in Europe by the Train Europe forum since 2002, it has always been held on the second Saturday in December at midnight to ensure that it does not contain any deadlocks. Secondly, in the event of delays or diversions, it is the task of the dispatcher , partly supported by algorithms in the control software of digital interlockings , to recognize and prevent potential deadlocks. The software then contains an algorithm called "Overfill Prevention", which only allows routes to be set automatically if the track system is not overfilled as a result: For example, a fourth route is prevented from being set at the transfer point in track changing mode and a route leading away from it is thus kept free .

Such systems are implemented in the Lötschberg base tunnel and in the Gotthard base tunnel on the basis of the European Train Control System . However, this is technically only possible if the systems on both sides belong to the same interlocking area. In the other cases, the train notification procedure , in which the abandonment of a journey into a section with bidirectional operation must be agreed in advance by offering and accepting between the dispatchers involved, serves as protection against getting stuck.

Theoretical description

Each section (highlighted in red) can only be occupied by one train that is waiting according to the blue arrow to enter the next block section. The deadlock can be recognized by the fact that the directed graph of these arrows has a cycle .

The same four conditions apply to the occurrence of a deadlock in the railroad as for a deadlock in computer science . The first three criteria are always met due to the structure of the railway or train protection.

  1. Each train sequence section (block section) can only be occupied by one train and is then blocked for others (" mutual exclusion ").
  2. Each train waits until it can enter the next block section and only then releases the previous track (“hold and wait”).
  3. No trains can be removed from the system (“No Preemption ”).
  4. There is a "waiting chain" that a train has to enter a block section which, due to a circular reference, can only become free after the train has left its own block section.
A loop occupied by a train with several train heading sections would be a hypothetical rail infrastructure in which deadlocks cannot occur.

The decisive fourth criterion depends on the operating situation and the structural conditions, i.e. the multiple tracks , the availability of sidings and the size of the stations. In a somewhat more complex railway system with single-track lines or track changing operations, it is impossible to prevent deadlocks in principle. In contrast to software processes, which can basically be aborted and restarted, it is normally impossible with the railways to take trains off the track at short notice and "restart" them. The only way out is to prevent deadlocks through foresight and appropriate control. For the automated train operation as well as for the simulation and determination of the maximum capacity of railway systems special algorithms are necessary, which exclude deadlocks and enable an efficient railway operation. By means of computational complexity can be calculated that for an increasing number of features and block sections the necessary computing power increases sharply: the u of Dessouky. a. examined algorithm is NP-hard .

Web links

Individual evidence

  1. Ulrich Maschek: Eisenbahnsicherungstechnik , Section 13.4.4.3 Counter-run protection . In: Lothar Fendrich (Ed.): Handbuch Eisenbahninfektur , 2007, pp. 599–648, here p. 630, doi: 10.1007 / 978-3-540-31707-4 .
  2. Jacob carbon Russ: investigation of methods to avoid deadlocks in synchronous railway simulation programs . Diploma thesis, Institute for Traffic Management, University of Applied Sciences Braunschweig / Wolfenbüttel, 2007, p. 5, 27–31.
  3. a b c Jörn Pachl : System technology of rail traffic: plan, control and secure rail operations . 6th edition, Vieweg + Teubner 2011, p. 214. ISBN 978-3-8348-1428-9 , doi: 10.1007 / 978-3-8348-8307-0 .
  4. ^ A b Yong Cui: Simulation-Based Hybrid Model for a Partially-Automatic Dispatching of Railway Operation . Dissertation, University of Stuttgart, 2009, p. 55 ff.
  5. a b c Jörn Pachl : Avoiding Deadlocks in Synchronous Railway Simulations . In: 2nd International Seminar on Railway Operations Modeling and Analysis , Hannover 2007, urn : nbn: de: gbv: 084-12898 .
  6. Jacob carbon Russ: investigation of methods to avoid deadlocks in synchronous railway simulation programs . Diploma thesis, Institute for Traffic Management, Braunschweig / Wolfenbüttel University of Applied Sciences, 2007, p. 7.
  7. Christian Hellwig, Dagmar Wander: Through the mountain at high speed - ETCS level 2 in the Lötschberg base tunnel . In: Signal und Draht 96 (10), 2004, pp. 14-17.
  8. Generations and Centuries Project . Article about the Gotthard Base Tunnel on siemens.ch. Retrieved December 15, 2019.
  9. ^ Sue Morant: Novel traffic safety systems keep Gotthard trains moving . In: International Railway Journal , June 16, 2016.
  10. P. Cazenave, M. Khlif-Bouassida, A. Toguyéni: Collisions avoidance and deadlocks prevention, for dynamic routing of trains in a railway node . IEEE (Ed.): 2019 6th International Conference on Control, Decision and Information Technologies (CoDIT) , doi: 10.1109 / CoDIT.2019.8820580 .
  11. Maged M. Dessouky, Quan Lu, Jiamin Zhao, Robert C. Leachman: An exact solution procedure to determine the optimal dispatching times for complex rail networks . In: IIE Transactions 38 (2), 2006, pp. 141-152, doi: 10.1080 / 074081791008988 .