Basic Operation
Most ABS systems use three or four block arrangements where an obstruction in block 1 will prompt a warning entering block 2 and allow full speed for trains entering block 3. In instances where blocks are short or higher capacity is needed 4 (or more) blocks are used where trains are then given multiple warnings of an impending obstruction. For basic block status the Red/Yellow/Green system is nearly universal with Red indicating an obstructed block, yellow indicating an obstructed block is approaching and green indicating that no obstruction is to be expected.
The most common way that ABS systems detect track occupancy (by a train or obstruction) is by passing a low-voltage current through the track between the signals and detecting whether the circuit is closed, open, or shorted. A train's metal wheels and axles will pass current from one rail to the other, thereby shorting the circuit. If the ABS system detects that the circuit is shorted between two signals, it understands that a train, or obstruction is occupying that block and will "drop" the signals (display a restricting or stop indication) on either side of that block to prevent another train from entering (if the block is governed by a positive stop). In the United Kingdom the system is referred to as Track Circuit Block (TCB) to avoid confusion with the use, in that country, of the acronym AB for Absolute Block.
ABS system electronics are also able to detect breaks in the rail or improperly-lined switches (if the switch is established in the circuit), which result in an open circuit. These will also cause the signal's aspect to 'drop', preventing any trains from entering the block (if the signal system prevents it), and running the risk of a run-through switch or derailment.
Read more about this topic: Automatic Block Signaling
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