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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 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 technology has existed for decades, the impracticality of early systems 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]}

"Moving block" is not technically a standard, rather it is a concept that can be implemented through multiple standards.

CBTC is the most common associated standard, however CBTC as it is described in IEEE 1474 (1999)^[7] makes no mention of a requirement for moving block operation. That said, the overwhelming majority of moving block systems use a signalling system consistent with the IEEE 1474 (1999) standard. Many different manufactures create systems consistent with the IEEE 1474 standard, and very few of them (if any) are compatible with each other.^[2]

• Thales Seltrac^[8]
• Bombardier CITYFLO (various versions)
• Alstom Urbalis^[9] (various versions)
• Siemens Trainguard MT CBTC^[10]
• Nippon Signal SPARCS^[11]
• Invensys Sirius^[12]
• Argenia SafeNet CBTC^[13]
• CAF Optio CBTC Option Solution

Transmission-based Train Control (TBTC) is an earlier form of CBTC that used induction loops on the track for communication with the signalling system, rather than radio signals or some other method.^[14] The words Transmission and Communication and synonyms in some circumstances, so neither one of these names accurately describes what each standard is. List of systems considered to use TBTC are:

• London Underground Jubilee Line (CBTC was used on the extension to Battersea Power Station)
• Docklands Light Railway
• Positive Train Control The US standard for a modern train protection system is considered by some to be an example of TBTC, however it potentially is better described as an Automatic Train Protection system though "advanced" versions exist that allow for moving block signalling.^[15]

ETCS is the signalling protocol for the European Rail Traffic Management System (ERTMS). This system is commonly known to have three levels: Level 1 (an ATP system only); Level 2 (a virtual block system that can also be provided with Automatic Train Operation (ATO)); and Level 3 (similar to Level 2 but uses moving block and can do away with a lot of the lineside equipment. In practice level 3 is not yet used, and this has become an extension of Level 2.^[4] Equipment is produced by various manufactures, but this standard has protocols and therefore all ETCS equipment is compatible, unlike CBTC systems.^[2]

Theoretically moving block can provide capacity advantages compared to fixed block systems, however in practice such advantages are difficult to fully realise.^[16]

The main reason for this is a combination of the way railway networks practically operate, and tolerances within the moving block system.^[2]

While a moving block system can technically allow a train to get as close as it can to the train in front while still retaining enough space for it to be able to stop (using regular service brakes) should a further update to the movement authority not be received, in practice if a train was to drive this close to the train ahead, the tiny inconsistency between the movement authority updates would require frequent braking applications and likely result in the train naturally tending to travel further behind. Most moving block systems also operate with a buffer to account for this, so trains might be 10 to 30 metres off the ideal, or "perfect" positioning.^[1][4][16] This helps to account for the transmission delays, and the slight inconsistency in train positioning calculations. Additionally, transmission between the train and the signalling system isn't literally continuous, instead it is sent as packages of information on the order of several times per second, to as infrequently as several seconds between transmissions.^[17] What this means is in practice, is that movement authority is given as several metre sections at a time, often with a buffer and a slight delay from the actual position of the train ahead. Therefore, a similar level of performance could be achieved using fixed, but very small blocks. This is in fact how the Moscow Metro, and London Underground Victoria Line operate. They do not have moving blocks, merely a very high number of closely spaced "virtual" blocks. These networks are often considered to be two of the highest capacity railway lines in the world.^[18]

The second reason why capacity is not necessarily improved, is because trains operating on a railway line with stations must make station stops. This time spent in a station means trains won't travel anywhere near as close to each other on 95% of the railway as they technically would be able to, if there were no stations. Consider that a two-track railway with four parallel platforms (2 per direction) at stations can have more or less double the frequency of the same line, but with only two platforms at stations (one per direction) even if both lines use equivalent signalling systems.^[19] This reality means that most of the benefits of a moving block signalling system can only be achieved in and around stations. However, then consider that almost all railways have an operational requirement that a following train cannot begin to enter the train platform, until the rear of the previous train has completely departed.^[20] This acts as a "fixed" block even on moving block systems,^[21] and will necessarily limit the throughput of the line to only that which is possible using conventional signalling practices. Most of the benefit networks gain from using moving block actually comes from the increased consistency of train movement, one gets from ATO. However, ATO is possible even without moving block.^[22]

Moving block can increase the capacity of a line if this limitation is removed from the system, which purportedly has been done on some railway networks, such as the Hong Kong MTR and at some stations, under certain conditions on the New York City Subway's BMT Canarsie Line (L train), however there is no verification of this available. Additionally, if it was permissible to give the following train movement authority past the rear of the leading train (up to the point where the rear of the leading train would end up if its emergency brakes were applied) capacity could be further increased.^[4] However, this has never been done and is currently considered unsafe.^[4]

Instead, the advantage of Moving block systems generally is that of decreased lineside equipment, which can save money in comparison to achieving the same headway capacity using the large amount of additional equipment it would take to do it with fixed or virtual block systems.^[2]

Moving block is in use on several London Underground lines, including the Jubilee, and Northern lines, and parts of the sub-surface lines.^[23] In London it is also used on the Docklands Light Railway^[24] and the core section of the Elizabeth line.^[25] New York City Subway's BMT Canarsie Line (L train), Tren Urbano (Puerto Rico),^[26] Singapore's MRT, and Vancouver's SkyTrain, also employ moving block signalling. It is also used by the Hong Kong MTR, on the Tuen Ma line, Disneyland Resort line, South Island line and the East Rail line .^[27]

It was supposed to be the enabling technology on the modernisation of Britain's West Coast Main Line which would allow trains to run at a higher maximum speed (140 mph or 230 km/h), but the technology was deemed not mature enough, considering the large number of junctions on the line, and the plan was dropped.^[21] Current implementations of Moving block have only been effectively proven on segregated networks with few junctions. The European Rail Traffic Management System's level-3 specification (naming on this has recently changed)^[4] for the European Train Control System, aims to provide a more robust version of moving block which can work with complex railways, however the difficulty in achieving this means that the system has not yet been implemented.^[4]