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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.
Like the manual block systems outlined above, automatic systems divide the route into fixed blocks. At the end of each block, a set of signals is installed, along with a track-side sensor. When a train passes the sensor, the signals are triggered to display the "block occupied" aspect on the signals at either end of that block.
In Canada, non-signalled 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 RTC.
The invention of train detection systems such as track circuits allowed the replacement of manual block systems such as absolute block with automatic block signalling. 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.
In 1906, the Great Western Railway in the UK developed a system known as "automatic train control". In modern terminology, GWR ATC is classified as an automatic warning system (AWS). This was an intermittent train protection system that relied on an electrically energised (or unenergised) rail between, and higher than, the running rails.
CBTC is a signalling standard defined by the IEEE 1474 standard. [1] The original version was introduced in 1999 and updated in 2004. [1] The aim was to create consistency and standardisation between digital railway signalling systems that allow for an increase in train capacity through what the standard defines as high-resolution train location determination. [1]
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. These are much less common than the three ...
The system also has automatic and manual "key-by" red signals where the conductor can insert a physical key into the wayside block signal, changing the signal indication from red to yellow. The signals involve the operation of an automatic stop with an automatic or manual release, then to proceed with caution, with preparations to stop in case ...