Moving block
In railway signalling, a moving block is a signalling block system under which the "blocks", or safe zones around trains, are defined in real time by computers. That requires both the knowledge of the exact location and the speed of all trains at any given time, as well as continual communication between the central signalling system and the signalling system in the train cab.
Moving block allows trains to run closer together (reduced headway) while maintaining the required safety margins, thereby increasing the line's overall capacity. It can 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 transmit back the permitted operating speed to enable the required flexibility. The European Train Control System (ETCS) also has the technical specifications to allow moving block operations, though no system uses it currently, apart from test tracks. Information about train location can be gathered through active and passive markers along the tracks, as well as train-borne tachometers and speedometers. Satellite-based systems are not used because they cannot work in tunnels.
Traditionally, moving block works by having a series of transponders in the rail corridor that have a known location. When a train travels over a transponder, it will receive the identification information allowing the train to know precisely where on the network it is. Because trains also have the ability to determine their own speed, that information can be combined and transmitted to the external signalling computer at a rail operations centre.
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 calculate exactly where the train is, even if it is between transponders. That allows the signalling system to give a movement authority to a following train, right up to the rear end of the preceding train. As more information comes in, that movement authority can be continuously updated, thereby achieving the "moving block" concept. Each time a train passes a transponder, it re-calibrates the location allowing the system to retain accuracy.
Technologically, the three most difficult parts to achieve a moving block railway system are:
- Continuous communication between a signalling system and all trains.
- Proving the train is intact
- Reliability
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. 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.
Train integrity (proving the train remains intact), 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). The only way a moving block system knows where a train is, is from the train's own identification of where it is. Traditionally, signalling systems use external means, such as axle counters and track circuits, to determine the location of a train. What that means is that most trains have no way of positively confirming that the entire train is still connected. Such systems can easily be added to multiple-unit passenger trains, especially if their cars are very rarely separated, but the implementation of technology to do the same with locomotive-hauled trains is significantly more involved. Every effective solution requires expensive technology, the cost of which may outweigh the benefits of the moving block system.
Another version of the moving block system would be to locate computers solely on the trains themselves. Each train would determine its location in relation to all the other trains, and set its safe speeds using that data. Less wayside equipment is required compared to the off-train system but the number of transmissions is much greater.