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Signals are erected beside railway lines to give drivers of approaching trains information about the state of the line ahead. Although a signal may be visible to drivers of trains passing on more than one line, every signal applies only to one specific line and to movements in just one direction. Signals are usually, though not always, placed immediately to the left of the line to which they apply.
Almost all signals are equipped to show two or more different indications (or 'aspects'). The meanings of the various signal indications are defined in the Rule Book. The most important of all the indications is 'danger - stop', which commands the driver of an approaching train to stop. Signals that are capable of displaying a 'stop' indication are referred to as 'stop signals', as distinct from 'distant signals'. A 'proceed' indication is not an instruction to proceed but merely an indication that it is safe to proceed from a signalling point of view.
On any railway where trains follow one another along the same track, each train is separated from the next one (during normal working) by at least one signal displaying a 'stop' indication. The driver's observance of the 'stop' indication prevents the train colliding with the one in front of it. A stop signal cannot show a 'proceed' indication unless the line ahead is known to be clear at least as far as the next stop signal (plus, where required, a further safety margin beyond the signal, called an 'overlap') or buffer stop. This is the basis of the 'space interval' system, whereby the line is divided into sections, each of which may be occupied by just one train at a time. (Under the long-obsolete 'time interval' system, the line was assumed to be clear after a specified period of time had elapsed since the last train passed.)
|Fig. 1: Stop Signals to separate following trains.|
The more closely spaced the stop signals are, the shorter the sections and the better the capacity of the line to carry a higher volume of traffic.
Another important function of stop signals is to prevent trains running onto movable items of infrastructure unless they are in the correct position for the passage of the train. By far the most common movable infrastructure features are points; a less common example is a swing bridge. Stop signals positioned on the approach to converging junctions prevent conflicting movements by being interlocked such that only the signal reading from the route for which the points are set can be cleared. Stop signals also prevent conflicts where tracks cross one another on the same level, as well as at certain types of road level crossings. A single stop signal may protect a succession of features of the types mentioned. Signals provided to protect such features can, in most cases, also be used to separate following trains as shown in figure 1.
|Fig. 2: Stop Signals to protect infrastructure features.|
Automatic level crossings operate independently of the signalling system and do not require to have protecting signals.
Because a train running at speed takes a long distance to stop (perhaps in excess of one mile), a driver must have prior warning of a stop signal ahead. This is the function of the distant signal, provided on the approach to a stop signal (or a succession of stop signals) roughly at the point where braking should commence if the next stop signal is at 'danger'. Braking distance is dependent on permissible speed, gradient and the braking capability of trains that operate over the line concerned. If the distant signal is showing a 'clear' indication, the train can continue running at speed. If, however, a 'caution' indication is displayed, the driver must be prepared to stop the train at the stop signal ahead. A distant signal may be co-located with a stop signal.
|Fig. 3: Distant Signal on approach to a Stop Signal.|
Passing a stop signal at 'danger' without authority (an occurrence termed a 'Signal Passed At Danger' or 'SPAD') can have disastrous consequences. With this in mind, various systems have been devised to help drivers obey signals, principally the Automatic Warning System (AWS) and the newer Train Protection & Warning System (TPWS). Both systems, which are widely fitted on Britain's railways, also give some limited assistance in the observation of speed restrictions. The primary function of AWS is to sound an audible indication when approaching a distant signal and to apply the brakes if the driver fails to acknowledge a warning indication. TPWS intervenes to apply the brakes when a fitted stop signal is passed at 'danger' or the train is approaching at excessive speed. Automatic Train Protection (ATP) provides a more comprehensive level of supervision but is not yet widely deployed in Great Britain. In order to function, safety systems of the types mentioned require the fitment of special equipment on board trains as well as on the track.
|Fig. 4: Typical fitment of AWS and TPWS.|
The signals so far described on this page are 'main signals', for controlling movements along running lines. Other signals, categorised as 'shunting signals', control low-speed movements such as those to or from sidings or through crossover roads. Shunting signals, which do not need to be read at high speed, are smaller and less conspicuous than main signals and are usually mounted down at ground level. Clearance of a shunting signal authorises the driver to proceed cautiously as far as the line is clear, or to the next signal. Because shunting frequently involves movements onto lines that are already occupied by vehicles, clearance of a shunting signal does not give any assurance that the line ahead is clear. The driver must understand that the line ahead may be occupied and must be prepared to stop short of any obstruction. A shunting signal may be co-located with a main signal, in which case clearance of the shunting signal authorises the driver to pass the main signal at 'danger', and vice versa.
|Fig. 5: Example layout with Shunting Signals.|
Cab signalling is a system that puts the signal indications on a display inside the driving cab, thereby dispensing with lineside signals. Two such systems are TVM430, as used on the Channel Tunnel Rail Link (between London and the Channel Tunnel), and ERTMS Level 2, which was introduced initially on the Cambrian Lines. Fixed marker boards are placed at the lineside in lieu of signals, to mark the start of each section. Cab signalling is a necessity on high-speed lines, where permissible speeds may be in excess of 200 km/h (125 m.p.h.), because drivers might not have sufficient time to reliably read and interpret a lineside signal's indication.