Railway signalling block systems

Signalling block systems enable the safe and efficient operation of railways by preventing collisions between trains. The basic principle is that a track is broken up into a series of sections or "blocks". Only one train may occupy a block at a time, and the blocks are sized to allow a train to stop within them. That ensures that a train always has time to stop before getting dangerously close to another train on the same line. The block system is referred to in the UK as the method of working, in the US as the method of operation, and in Australia as safeworking.

Communications-based train control (CBTC) is a railway signaling system that uses telecommunications between the train and track equipment for traffic management and infrastructure control. CBTC allows a train's position to be known more accurately than with traditional signaling systems. This can make railway traffic management safer and more efficient. Rapid transit systems (and other railway systems) are able to reduce headways while maintaining or even improving safety.

In railway signalling, a moving block is a signalling block system where the blocks are defined in real time by computers as safe zones around each train. This requires both knowledge of 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 (reduced headway) while maintaining required safety margins, thereby increasing the line's overall capacity. It may be contrasted with fixed block signalling systems.

Train order operation is a system for safely moving trains using train orders, as opposed to fixed signals or cab signalling. In train order operation, a "train order" is an order issued by or through a proper railway official to govern the movement of trains.

The railway block signalling bell code is a system of bell sounds used in Great Britain to communicate between manually operated Signal Boxes in implementing the railway block system. (The bell system is not used in modern power signal boxes, other than to any older adjacent signalboxes.) Each such signal box has a bell circuit to the boxes on either side of it along the line. The equipment consists of a plunger or tapper (rather like a Morse key) which when pressed, rings a single-stroke bell in a neighbouring box. That box similarly has a tapper for communicating back, so boxes have keys each of which rings the bell in a neighbouring box. The bells sound different tones, so that the signalman can tell them apart by ear.

Radio Electronic Token Block is a system of railway signalling used in the United Kingdom. It is a development of the physical token system for controlling traffic on single lines. The system is slightly similar to North American direct traffic control, which unlike RETB does not have a cab display unit.

Centralized traffic control (CTC) is a form of railway signalling that originated in North America. CTC consolidates train routing decisions that were previously carried out by local signal operators or the train crews themselves. The system consists of a centralized train dispatcher's office that controls railroad interlockings and traffic flows in portions of the rail system designated as CTC territory. One hallmark of CTC is a control panel with a graphical depiction of the railroad. On this panel, the dispatcher can keep track of trains' locations across the territory that the dispatcher controls. Larger railroads may have multiple dispatcher's offices and even multiple dispatchers for each operating division. These offices are usually located near the busiest yards or stations, and their operational qualities can be compared to air traffic towers.

A track warrant is a set of instructions issued to a train crew authorizing specific train movements. The system is widely used in North America. The warrant is issued by the train dispatcher and delivered to the train crew via radio. The train crew copies the instructions onto a pre-printed paper form and reads back the warrant to ensure that nothing was misunderstood.

Positive train control (PTC) is a family of automatic train protection systems deployed in the United States. Most of the United States' national rail network mileage has a form of PTC. These systems are generally designed to check that trains are moving safely and to stop them when they are not.

In railway signalling, a token is a physical object which a train driver is required to have or see before entering onto a particular section of single track. The token is clearly endorsed with the names of the section to which it belongs. A token system is more commonly used for single lines because of the greater risk of collision in the event of a mistake being made by a signaller or traincrew than on double lines.