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Semi-automatic signals do not have a number plate, but can display an explicit "restricted proceed"-type signal. Other types of signals include train order signals , manual block signals or signals governing special safety appliances such as slide fences , non-interlocked sidings , road crossings, etc.
In Canada, non-signaled and Automatic Block Signal (ABS) territories are operated according to OCS rules. The only authority for movement in OCS territory is possession of a Clearance or Track Occupancy Permit (TOP) While the line might be equipped with signals (ABS), these signals are unsupervised and uncontrolled by the Rail Traffic Controller.
Such signals as electrically operated automatic and interlocking signals were widely adopted in the U.S. After 1908 with the advent of the Union Switch & Signal 's Styles "B" & "S" and the General Railway Signal Company's universal Model 2A mechanisms manual semaphores and earlier electric disc signals quickly began to disappear.
Automatic block signaling (ABS), spelled automatic block signalling or called track circuit block (TCB [1]) 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.
Highball Signal – Historic railroad signal in Delmar, Delaware, US; North American railroad signals; Railway signalling – The principle of signals used to control railway traffic; Train protection system – Railway fail-safe against human error; Train speed optimization – A system that reduces the need for trains to brake and accelerate
Under automatic block signalling, signals indicate whether or not a train may enter a block based on automatic train detection indicating whether a block is clear. The signals may also be controlled by a signalman, so that they only provide a proceed indication if the signalman sets the signal accordingly and the block is clear.
A semi-automatic signal can be recognized by its post which is painted with red and white strips. Dwarf versions have their boxes painted so. A red (stop) aspect on a semi-automatic signal must not be passed. As presented on this compact chart, semi-automatic signals can display both near and distant functions.
Automatic block signaling uses a series of automated signals, normally lights or flags, that change their display, or aspect, based on the movement of trains past a sensor. This is by far the most common type of block system as of 2018, [update] used in almost every type of railway from rapid transit systems to railway mainlines.