Re three position Low Speed signals . Low Speed signals take you from either a station to a siding , a siding to a station , a running line to a running line , a siding to a siding , or into a crossing loop . They may also be fitted to a Home Signal to bring a train into an occupied road , as a calling on signal.
Where a Low Speed signal is on a Home Signal ( which protects Stations , Sidings , Signal Boxes , Level Crossings and Junctions ) the Home Signal will display Red over Red in a vertical line.
The Low Speed will be non illuminated black. Once the Low Speed Signal is initiated , either by the Signalman , or by a timing circuit that ensures the train is either at, or nearly at, stop ; the Low Speed will illuminate a small Yellow. The Low Speed light ( always a light when mounted on a Home Signal ) is vertically below the two red lights.
Home signals may also be equipped with an illuminated route indicator , usually to the left of the Low Speed ( there are exceptions ) . The indication is usually a letter , sometimes an arrow . In the event of there being more than one route indicator on the post they will be fitted adjacent to each other and below the Low Speed ( there are exceptions ).
Best wishes and regards, Radioman