From: clive@x.co.uk (Clive Feather)
Newsgroups: rec.railroad
Subject: Re: Signal type (was: Re: Semaphore Signals)
Date: 9 Oct 92 12:33:10 GMT
Organization: IXI Limited, Cambridge, UK

In article <1992Sep2.005414.19229@mailhost.ocs.mq.edu.au> eoliver@laurel.ocs.mq.edu.au (Eddie Oliver) writes:
>In article <28144@olympus.wustl.edu> jps@dworkin.wustl.edu (James Sterbenz) writes:
>> This is a very important distinction emphasising that this is a true 
>> distant signal (unusual in the US these days), and unlike other 
>> (combination home/distant) signals ONLY repeats the (home) interlocking
>> signal.  There is no indication of the track between the signal and the
>> interlocking and the engineer cannot assume that the track is clear
>> based on the signal (there will be other ways to make this assumption)
> Suppose now that a train is stopped at a certain signal "B". The signal
> immediately behind it is the distant which is at caution, and the previous
> "normal" signal "A" is at stop. The layout would be like
>
> Signal A at stop --------  Distant at caution --- stopped train -- signal B.
>
> If a train  passes signal A in the stop position (legally or illegally), the
> next signal it sees is the distant AT CAUTION. If the engine crew forget that
> this is indeed a distant, they may interpret it as giving a clear path to
> the next signal. In fact there is a rather large obstacle in the way, and
> much damage can result.
> The obvious prevention measure is to hold the distant at stop while the
> track circuit is occupied between it and the signal it repeats. While
> this is now being done (100 years too late) in new installations, there are
> still countless accidents waiting to happen.

Firstly, it must be remembered that for many years signals were
mechanical, and adding an extra aspect to a distant signal was an
expensive matter. Obviously this is less true for coloured-light
signals.

Secondly, in most layouts (particularly in the UK), the distance from
stop signal to the next distant is much longer than the distance from
the distant to the stop, and so there is much more likely to be a train
before the distant (where adding a danger aspect would not help) than
after (where it would). Thus such a change is probably not cost-effective.
Furthermore, except in an emergency, there would be at least one distant
signal at danger before your signal A.

There used to be one installation in the UK where distant signals were
provided with a danger aspect: the LNWR (later LMS, later LM region)
"new lines" from Camden to Watford Junction, plus branches. This used
the following aspects at stop signals:

        G                       R                       R
        |   Proceed,            |   Danger,             |  Proceed
        |   line clear          |   stop and            |  cautiously,
        O                       R   stay here.          y  train ahead
        |                       |                       |  before next
        |                       |                       |  stop signal

and the following aspects at distant signals:

        G                       Y                       R
        |   Next stop           |  Next stop            |  Train before
        |   signal is           |  signal is            |  next stop  
      O-+   green             O-+  red                R-+  signal, wait
        |                       |                       |  60 seconds 
        |                       |                       |

[y = small yellow light, O = head not lit]
The right hand aspect at stop signals appeared after the train had been
waiting for 60 seconds, provided that the line was clear to the next
distant signal: this would occur if there was a train between that
distant and the next stop signal. If that train had not moved by the
time the following train reached the distant, the latter had to wait 60
seconds, sound one long blast on the horn, and then proceed cautiously
up to the rear of the other train. As a result, it was possible to end
up with several trains nose-to-tail waiting at a stop signal.

This route also had interesting junction distant signals:

        G                       Y                       Y   
        |     Straight          |     Right-hand        |     Left-hand
     O--+--O  ahead          Y--+--O  diverging      O--+--Y  diverging
        |     route             |     route             |     route
        O     clear             Y     clear             Y     clear
        |                       |                       |   
        |                       |                       |   

The last two are *not* the wrong way round.
-- 
Clive D.W. Feather     | IXI Limited         | If you lie to the compiler,
clive@x.co.uk          | Vision Park         | it will get its revenge.
Phone: +44 223 236 555 | Cambridge   CB4 4RZ |   - Henry Spencer
Fax:   +44 223 236 550 | United Kingdom      |
Newsgroups: rec.railroad