Moving block
Moving block allows trains to run closer together (reduced headway) while maintaining required safety margins, thereby increasing the line's overall capacity.
Information about train location can be gathered through active and passive markers along the tracks, and train-borne tachometers and speedometers.
[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] Each time a train passes a transponder, it re-calibrates the location allowing the system to retain accuracy.
[2][4] Traditionally signalling systems use external means, such as axle counters and track circuits to determine the location of a train.
[4] Every effective solution would require expensive technology, the cost of which may outweigh the benefits of a moving block system.
[16] The main reason for this is a combination of the way railway networks practically operate, and tolerances within the moving block system.
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.
[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.
Most of the benefit networks gain from using moving block actually comes from the increased consistency of train movement, one gets from ATO.
[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.
[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.
[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.
