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.