It's also worth pointing out that traditional single light colour light signals can only display stop, caution, medium or proceed (or turnout, or "low speed" or "calling on" etc with the addition of subsidiary lights). A double light colour light signal can also show preliminary medium, which is used when on steep grades, assisting heavier trains in managing their speed when approaching a slow turnout or restricted signals. This is used to good effect when descending from Sutherland to the bridge over Georges River, and when descending from Beecroft towards Epping.
Another advantage of 5 aspect signalling is that it reduces the ambiguity of repeated mediums.
As discussed in another thread, at North Strathfield, when approaching the junction to the Up North Suburban Flyover, there are two G/Y mediums in a row. This is because the distance between the junction Home signal and the first medium (G/Y) is less that braking distance, and a second medium (G/Y) is required to maintain adequate braking distance.
North Strathfield Flyover junction dates from 1980s, before the Preliminary Mediums (Green over Pulsating Yellow) was defined. Sooner or later G/PY aspects will be provided here.
Ambiguous signals are undesirable because they are ambiguous and have more than one meaning.
Another ambiguity is the Medium aspect (G/Y) which has two meanings:
* Next signal at Caution (G/R) and signal within minimum braking distance is a Stop.
* Next signal at Turnout (Y/R or Y/Y).
** The braking curves and distances may be different for the above two situations.
This ambiguity is being resolved by introducing the "Junction Indicator (JI)", a white diagonal bar above the top Green:
* (G/Y) = next signal at Caution
* (W/ over G/Y) = next signal at Turnout.
* The White Bar is lamp proved before the Bottom Yellow is lit.
** Also, the driver gets more warning if the wrong route is set.
** The oldest JIs at Cabramatta and Merrylands use BR-style 5 white light "feathers".
A good place to see the Junction Indicator is at Birrong since alternate trains generally alternate between the straight and turnout aspects. The new G/Y signal at Concord West approaching the NSFC dive at North Strathfield has a JI.