Other Signals
Preliminary Routeing 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 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.
Read more about this topic: UK Railway Signalling
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