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The basic principle is that a track is broken up into a series of sections or "blocks". Only one train may occupy a block at a time, [citation needed] and the blocks are sized to allow a train to stop within them. [1] That ensures that a train always has time to stop before getting dangerously close to another train on the same line.
A mechanical lever frame inside the signal box at Knockcroghery in Ireland Waterloo station A signalbox, LSWR (Howden, Boys' Book of Locomotives, 1907). Mechanical railway signalling installations rely on lever frames for their operation to interlock the signals, track locks [1] and points to allow the safe operation of trains in the area the signals control.
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.
British lower-quadrant semaphore stop signal (absolute) with subsidiary arm (permissive) below A block signal and a 130 km/h speed limit at kilometer post 547 in Iisalmi, Finland Trains cannot collide with each other if they are not permitted to occupy the same section of track at the same time, so railway lines are divided into sections known ...
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?"
The signal box can also see the lines to and from Manchester and Sandbach and all the platforms on the north side of the station. The signal box, still at its original site, is located in Crewe Heritage Centre where there are now demonstrations and simulations of the signalling in the North Junction box by a team of signallers. [2]
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.