Moving block
In railway signalling, a moving block is a signalling block system where the blocks are in real time defined by computers as safe zones around each train. This requires both the need to know 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, while maintaining required safety margins, and thus increasing the line's overall capacity. Information about the location can be gathered through active and passive markers along the tracks, and train-borne tachometers and speedometers. Satellite-based systems cannot be used because they will not work in tunnels.
Implementation
Urban
Moving block is in use on several London Underground lines including the Victoria line, Jubilee line, parts of the Northern line as well as the Docklands Light Railway. New York City Subway's L train, Singapore's North East Line, Circle Line, Downtown Line and Vancouver's Skytrain, also employ Moving Block signalling.
Inter-city
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. It forms part of the European Rail Traffic Management System's level-3 specification for future installation in the European Train Control System, which will at level 3 feature moving blocks that allow trains to follow each other at exact braking distances.
Communications-based train control
Nowadays, with the current radio-based Communications-based train control (CBTC) systems using the moving block principle, to reduce headways and increase transport capacity, it is very much a reality.