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Absolute – Absolute signals are usually connected to an interlocking controlled by a block operator or train dispatcher. Their most restrictive aspect is "stop" and trains cannot pass them at stop unless they obtain special authority.
Some railroads, notably the Pennsylvania Railroad (PRR), had a system of manual block signals activated by wayside operators in stations or interlocking towers eliminating the need for some trains to stop.
A block instrument on the Midland Railway. 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".
CTC automatic block signals along the Union Pacific Railroad Yuma Subdivision, Coachella, California. CTC makes use of railway signals to convey the dispatcher's instructions to the trains. These take the form of routing decisions at controlled points that authorize a train to proceed or stop.
Automatic block signaling (ABS), spelled automatic block signalling or called track circuit block (TCB) in the UK, is a railroad communications system that consists of a series of signals that divide a railway line into a series of sections, called blocks. The system controls the movement of trains between the blocks using automatic signals.
On a rail transport system, signalling control is the process by which control is exercised over train movements by way of railway signals and block systems to ensure that trains operate safely, over the correct route and to the proper timetable.
Every train carries an electronic unit that receives and sends an encrypted block of data which represents the token. The system is designed so that the control centre cannot issue a new token for a section of line until the current one is 'returned'.
Traditional signalling systems detect trains in discrete sections of the track called 'blocks', each protected by signals that prevent a train entering an occupied block. Since every block is a fixed section of track, these systems are referred to as fixed block systems.
Some of the fundamental principles of interlocking include: Signals may not be operated to permit conflicting train movements to take place at the same time on set route. Switches and other appliances in the route must be properly 'set' (in position) before a signal may allow train movements to enter that route.
In the U.S., semaphores were employed as train order signals, with the purpose of indicating to engineers whether they should stop to receive a telegraphed order, and also as simply one form of block signalling.