Re: Catchpoints - what are they?

[signal_spotter@my-deja.com]

In article <3931e81f@pink.one.net.au>,
  "Bradley Torr" <btorr@bigpond.nospam.com> wrote:
>
> signal_spotter@my-deja.com wrote in message <8gs4q4
$8qf$1@nnrp1.deja.com>...
>
> >Catchpoints are a set of points where only one rail is switched. They
> >are usually placed at the ends of loops (except CTC), sidings etc to
> >prevent vehicles from fouling the main line. They also serve the
> >purpose of allowing a valid overlap of the signal in rear. In other
> >words, the signal in rear can be cleared to a caution if the
catchpoint
> >is open and there is a train or the route is set for the main line.
> >This allows more train movements ie simultaneous entry into a main
line
> >and a loop line.
>
> Ian --
>
> I am sure that what you said above was extremely useful information,
and I
> thank you for your time in writing a response to my question. Could
you
> please take the trouble to write it in laypersons English though? I'm
a
> relative novice when it comes to railway terminology - until a few
months
> ago I referred to pantographs as "those bouncy things on top of
trains which
> touch those overhead wires", which should give some indication as to
how
> much of a novice I am. I'm learning new things about railways every
time I
> open this newsgroup up daily; of course you weren't to know what a
greenhorn
> I am - for me to decipher your message, you'll have to treat me as a
five
> year old ;-)
>
> Regards & thanks,
> Bradley.
>
>
Fair enough. Well basically an overlap is the margin of error after a
signal for a train travelling up to that signal should it pass it at
stop. It's worked out at the distance a train will come to a stand
after passing the signal at stop using emergency braking at it's full
line speed. In the metrop area this can be worked out easily because
electric trains will trip the trainstop and the emergency brakes will
be automatically applied. In country areas we allow about 300-400 m. So
what that means in laymans terms is that signals detect the track
circuits clear and the points detected in the correct lie from the
signal to it's destination signal plus the overlap before they can show
a proceed. Now on straight lines this is quite easy to acheive. But in
complex station areas if there is a set of facing points in the overlap
then there will be more than one overlap ie one through the facing
points normal and one through the facing points reverse and so on. So
if that signal reads up to it's destination signal and there is a
catchpoint (which acts as a facing point) in the overlap then that will
give the signal 2 overlaps too ie one through the catchpoint reverse
and one through the catchpoint open. And since there are no track
circuits in the catchpoint open (other than the points track itself)
then with the catchpoint open we can clear a signal up to it's
destination. Confused!! Don't worry I've been designing this stuff for
20 years and I'm still learning. Anyway hope this helps.

regards Ian
Signal Design Engineer


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