Re: Three Position Signalling

["Notagunzel" <notagunzel@bigfoot.com>]

Tony Gatt <baulko@tig.com.au> wrote in message
news:37F42C90.D11BE8CB@tig.com.au...

> Christopher_Martin GORDON wrote:

> > Three Position Dwarf Signals:
> > For a three position dwarf signal (at Caution, yellow) is it once the
> > train passes the signal it will revert to stop? Or the back of the train.
> > (I am assuming here the signal is controlled by track circuit and lever)

> There is an interesting one in the loop sidings at Somerton on the Down end.

> The signal is marked as V6.

> When the points are set for the Blue Circle siding, it is controlled by Lever
> 8 in Somerton Box (via a pilot lever)
> and displays a Yellow light.

> When controlled from Centrol for Moves to the Main line, it is controlled as
> signal 6 and displays a green light.

> I would assume if the auto signal up the line was at stop, it would show
> yellow as well (although I have never seen this.)

It is very possible that the Auto next in advance must be displaying at least
Top Yellow for any train to depart Somerton Loop on the Down.  This is so a
train does't have to run under a Caution Aspect through a section.  Most if not
all sections on the NE SG have a sequence:

Home(R/G) --(long)--> Auto(R/Y/G) --(short)--> Home.

Longer sections can have intermediate Autos:

Home(R/G) --(long)--> Auto(R/Y/G) --(short)--> Auto(R/G) --(long)-->
Auto(R/Y/G) --(short)--> Home.

A Signal applying into a long section can only display Stop or Top Green, and a
Auto applying into a short section displays R/Y/G over R.  So, the autos
applying to a short section, i.e. displaying R/Y/G over R, are pseudo-distants,
in a NSW CTC scheme they would be Distants.  It also means, that Drivers don't
approach the 3 aspect autos at Stop except when there is a Fault.

There are, of course exceptions to this arrangement, especially around Grade
Crossings, (existing and removed), and I'm sure Rod would be the first to point
this out. |-)

--
Mr Notagunzel.
Rail Transportation Connoisseur.
notagunzel@bigfoot.com
(Waiting for any move at http://www.bigfoot.com/~notagunzel)