This is a "Low Speed" Indication. It is a way of getting trains closer
together near junctions. All signalling in NSW has a system where there are
two levels of protection for every train. Normally, this means two Red over
Red indications behind each train. Also acceptable is One Red over Red
Indication and a set of catchpoints. At major signalboxes, such as Sydenham,
Strathfield and Sydney, the Low Speed system is utilised, meaning that the
second Red over Red has a small Green light underneath to indicate that the
signal may be passed at a maximum speed of 15 km/h. If an electric train
exceeds this speed, the trip mechanism will operate and stop the train.
The upkeep of this is that a train can move up slowly behind another one, and
close up the gaps. It is especially useful for getting trains into platforms
when the line ahead is blocked.
-- David Johnson - Rail Services Authority trainman@ozemail.com.au trainman@railpage.org.au http://www.ozemail.com.au/~trainman