Automatic and semi-automatic railway signalling

The evolution of railway signalling to meet the demands of safety, and later of traffic capacity, has culminated in the modern automatic and semi-automatic systems, which are designed to overcome the defects of manual working. In addition they offer valuable advantages, as their use permits of increased speeds and augmented services, which increase the full-load capacity of a given line; while their flexibility allows of a ready adjustment of traffic control to traffic variations. An early system using train-controlled switches is briefly touched upon, and its weak points are noted. The track circuit, which forms the basis of all modern automatic signalling, is considered in its simplest form, and also as modified to suit electrified lines. Apparatus controlled by track circuits is described, together with additional schemes for its safe operation. The functions and chief features of semi-automatic working are dealt with, and the advantages peculiar thereto are pointed out. Finally, a description is given of some auxiliary apparatus designed to expedite the handling of traffic.