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

Moving block is in use on London's Docklands Light Railway, London's Jubilee Line, New York's L line and Vancouver's Skytrain. 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.

But nowadays, with the current radio-based CBTC systems using the moving block principle, to reduce headways and increase transport capacity is very much a reality.