page_title: UK railway signalling
section_title: Other signals
hierarchical_section_title: UK railway signalling / Other signals
caption_attribution_description: English: SPAD Indicator protecting the entrance to a single line section
context_page_description: The railway signalling system used across the majority of the United Kingdom rail network uses lineside signals to control the movement and speed of trains.
  The modern-day system mostly uses two, three, and four aspect colour-light signals using track circuit block signalling. It is a development of the original absolute block signalling that is still being used on many secondary lines. The use of lineside signals in Britain is restricted to railways with a maximum speed limit of up to 125 miles per hour. This is the maximum speed at which the train can travel safely using line-side signalling; if the train runs any faster, it will not be possible for the train driver to safely read colour-light signalling. Trains operating at speeds faster than 125 mph use an in-cab signalling system that automatically determines and calculates speed restrictions.
context_section_description: Preliminary Routing Indicators (PRIs) are installed on the approach to certain junctions. When the junction signal is displaying a 'proceed' aspect, the PRI will display an arrow. The arrow points up when the highest speed route is set. When a diverging route is set, the arrow points in the appropriate direction (mimicking the junction indicator on the junction signal). This advance indication gives the driver an opportunity to stop before the junction points, if wrongly routed. At the present time, PRIs are few in number, but they are likely to become more common.
  Proceed on Sight Authority is a new concept which introduces an additional aspect to allow the signalman to authorise drivers to pass signals when they are at red due to influences within the interlocking. The signal will notionally be used where the route setting and locking function is still proved to be operable but a function such as train detection or lamp proving of a signal ahead may be failed. The authority will allow the driver to pass the signal and proceed at a speed slow enough that they may stop short of any obstruction (in common with other degraded modes of operation) The term may be abbreviated to "PoSA".
  A "Proceed On Sight Authority" is only shown in conjunction with a red main aspect on a main signal and when operated the two white lights in the subsidiary signal flash on and off together to warn the driver he is proceeding under caution only as far as the line can be seen to be clear.
  Off Indicator: An illuminated off indication means the associated signal is showing a proceed aspect. These are mainly used at stations, for the benefit of the train-crew and platform staff. When the display is blank, it means that the associated signal is at danger. An illuminated indication CD (close doors) is an instruction to close the train's power-operated doors. An illuminated indication RA or R (right away) means that station duties are complete and the train may depart.
  A SPAD indicator is a separate indicator which may be positioned after a main signal where there is a likelihood of a serious collision at a junction if a SPAD (signal passed at danger) occurs at the main signal. SPAD indicators are mounted against a blue backplate or surround. They are normally unlit but following a SPAD they display a steady red light between two flashing red lights arranged vertically. Any driver who sees a SPAD indicator illuminated must stop their train immediately, even if they can see that the signal pertaining to their line is showing a proceed aspect.
caption: SPAD indicator protecting the entrance to a single line
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