System for signalling trains developed during the 19th century and since superseded by the track circuit block system. The block system is based on the use of block sections, in other words, the section of line between the last signal passed at one signalbox and the last stop signal at the next signalbox, and the principle is that only one train is allowed on the same line in one block at any time. The so-called permissive block system allows for closer intervals between trains on lines with lower traffic levels than those controlled by the absolute block system by allowing more than one train in a section on the same line at the same time.