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In the simplest case with three signal boxes on a stretch of line without junctions, the central of the three boxes will receive a request for a train to be accepted, the signaller will set any relevant points (turnouts) and signals and signal acceptance, and then request acceptance by the next signal box along the line.
The signal box provided a dry, climate-controlled space for the complex interlocking mechanics and also the signalman. The raised design of most signal boxes (which gave rise to the term "tower" in North America) also provided the signalman with a good view of the railway under his control. The first use of a signal box was by the London ...
The tuned circuit often incorporates the circuit to either apply the transmitted signal to the track or recover the received signal from the other end of the section. Consider a railway with two block sections as in the diagram. Section 1 has frequency A injected at the left-hand end and received at the right-hand end.
In the UK, the beginning of the end for classic "Control" came with the commissioning of large power signal boxes from the 1950s. A power signal box (PSB) often has a number of signalmen operating multiple electric or computerized signalling panels and large illuminated track diagrams showing wide areas of operation.
An example is the process of signalling a train in the up direction (from A to C) past a signal box B. The signal box in rear is A and the signal box in advance is C. [2] The block indicators at B are in the Normal position. The signalman at A "offers" the train to B by sending an "Is Line Clear?"
A particular signal box may control one or more stop signals on each running line. In a traditional mechanically signalled area in the United Kingdom, it is most common for a signal box to have two stop signals governing each line. The first reached by a train is known as the home signal.
Track circuits can automatically detect some types of track defect such as a broken rail. In the event of power restoration after a power failure, an axle counted section is left in an undetermined state until a train has passed through the affected section. A track circuited section immediately detects the presence of a train in section.
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