Moving block

The safety distance (safe-braking distance) between trains in fixed block and moving block signalling systems

In railway signalling, a moving block is a signalling block system where the blocks are defined in real time by computers as safe zones around each train. This requires both knowledge of the exact location and speed of all trains at any given time, and continual communication between the central signalling system and the train's cab signalling system. Moving block allows trains to run closer together (reduced headway) while maintaining required safety margins, thereby increasing the line's overall capacity. It may be contrasted with fixed block signalling systems.

Communications Based Train Control (CBTC) and Transmission Based Signalling (TBS) are two signalling standards that can detect the exact location of trains and to transmit back the permitted operating speed to enable this flexibility.[1] The European Train Control System (ETCS) also has the technical specifications to allow moving block operations, though no system is uses it currently, besides test tracks. Information about train location can be gathered through active and passive markers along the tracks, and train-borne tachometers and speedometers. Satellite-based systems are not used because they do not work in tunnels.

Traditionally, moving block works by having a series of transponders in the rail corridor that each have a known location.[2] When a train traverses over a transponder, it will receive the identification information allowing the train to know precisely where on the network it is.[2] Because trains also have the ability to determine their own speed, this information can be combined and transmitted to the external signalling computer (at a rail operations centre).[2] Using a combination of time and speed, the computer can add the time since the train passed the transponder, and the speeds it has travelled at during that time, to then calculate exactly where the train is, even if it is between transponders.[2] This allows the signalling system to then give a following train a movement authority, right up to the rear end of the first train.[2] As more information comes in, this movement authority can be continuously updated achieving the "moving block" concept.[2] Each time a train passes a transponder, it re-calibrates the location allowing the system to retain accuracy.[3]

Technologically, the three most difficult parts to achieve a moving block railway system are:

  1. Continuous communication between a signalling system and all trains.[2]
  2. Proving Train integrity[4]
  3. Reliability[4]

Moving block signalling could not effectively be implemented until the invention of reliable systems to communicate both ways between a train and a signalling system. While such technically has existed for decades, the impracticality of early technology a system made it unviable for many years. Pulse codes were used on the first version of the London Underground Victoria line's signalling system.[5][6] However, a pulse code two-way communication system using the computational technology at the time would have been complicated, so a fixed block system was used instead.[5]

Train integrity, while not a complicated problem on short suburban and metro lines, becomes a much more difficult problem when dealing with a variety of different train types, train lengths, and locomotive hauled trains (as opposed to Multiple Units).[4] The only way a moving block system knows where a train is, is from the train's own identification of where it is.[2][4] Traditionally signalling systems use external means, such as axle counters and track circuits to determine the location of a train.[2] What this means is that most trains have no way of positively confirming that the entire train is still connected.[2][4] Such systems can easily be added to multiple unit passenger trains, especially if they are very rarely separated, but the implementation of technology to do the same with locomotive hauled trains is significantly more involved.[4] Every effective solution would require expensive technology, the cost of which may outweigh the benefits of a moving block system.

Another version of the moving block system would be to locate computers solely on the trains themselves. Each train determines its location in relation to all the other trains and sets its safe speeds using this data. Less wayside equipment is required compared to the off-train system but the number of transmissions is much greater.[citation needed]

  1. ^ "Moving Block — The Theory". ATP Beacons and Moving Block. Railway Technical Web Pages. 17 November 2016. Archived from the original on 9 February 2009. Retrieved 17 November 2016.
  2. ^ a b c d e f g h i j Versluis, Nina D.; Quaglietta, Egidio; Goverde, Rob M. P.; Pellegrini, Paola; Rodriguez, Joaquin (2024-01-01). "Real-time railway traffic management under moving-block signalling: A literature review and research agenda". Transportation Research Part C: Emerging Technologies. 158: 104438. doi:10.1016/j.trc.2023.104438. ISSN 0968-090X.
  3. ^ "CBTC Moving Block Principle – Railway Signalling Concepts". 2022-06-03. Retrieved 2024-10-10.
  4. ^ a b c d e f Hansen, Dominik; Leuschel, Michael; Körner, Philipp; Krings, Sebastian; Naulin, Thomas; Nayeri, Nader; Schneider, David; Skowron, Frank (2020-06-01). "Validation and real-life demonstration of ETCS hybrid level 3 principles using a formal B model". International Journal on Software Tools for Technology Transfer. 22 (3): 315–332. doi:10.1007/s10009-020-00551-6. ISSN 1433-2787.
  5. ^ a b "Technical Meeting of the Institution" (PDF). The Institution of Electrical Engineers. 1966.
  6. ^ "Victoria Line ATO Page". www.trainweb.org. Retrieved 2024-10-10.