Clive D. W. Feather wrote:
In article . com,
TheOneKEA writes
In the case of Turnham Green, both lines do have home signals - the
EB line's homes are A631^A and A631^B, and the WB's are A630^A and
A630^B. Would the overlaps on these homes simply be longer than
usual, to allow a train to alternately stop or pass through at
linespeed?

Yes. Just as with a signal between stations. The overriding
principle is that a train stop hit at line speed should stop the
train before the point of danger.

Thanks, that was what I thought.

This doesn't make much sense. Are you saying that if a train passes
a red signal at linespeed or higher and gets tripped, the signal in
rear could change to green if the entire train manages to exit that
signal's overlap?

Yes. But, in that case, the situation will still be protected. Um,
let's see:

snip

Clear? Or have I answered the wrong question?

I understand now. But it doesn't seem sensible to place the signals
that close together, or only hold one signal in rear of the signal
protecting an obstruction at danger.

In article . com,
TheOneKEA writes
I understand now. But it doesn't seem sensible to place the signals
that close together,

Signal spacing is chosen to meet various requirements, including maximum
throughput of trains. For example, on LU there are usually several
signals approaching a station, because this allows a train to draw up
close as the previous train departs. This is *better* than spacing the
signals further apart, but could mean that there are four or five red
signals behind a train under some circumstances.

or only hold one signal in rear of the signal
protecting an obstruction at danger.

Why? If one red signal can protect the obstruction, what's the need for
more?

--
Clive D.W. Feather | Home:
Tel: +44 20 8495 6138 (work) | Web: http://www.davros.org
Fax: +44 870 051 9937 | Work:
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Clive D. W. Feather wrote:

Signal spacing is chosen to meet various requirements, including
maximum throughput of trains. For example, on LU there are usually
several signals approaching a station, because this allows a train
to draw up close as the previous train departs. This is *better*
than spacing the signals further apart, but could mean that there
are four or five red signals behind a train under some
circumstances.

I've seen numerous examples of this around the system and guessed that
the rationale was something similar to what you've just stated.


or only hold one signal in rear of the signal
protecting an obstruction at danger.

Why? If one red signal can protect the obstruction, what's the need
for more?

You just showed that under certain circumstances, one signal is not
enough to protect an obstruction (or at least I think you did...)

TheOneKEA wrote:
Clive D. W. Feather wrote:

Signal spacing is chosen to meet various requirements, including
maximum throughput of trains. For example, on LU there are usually
several signals approaching a station, because this allows a train
to draw up close as the previous train departs. This is *better*
than spacing the signals further apart, but could mean that there
are four or five red signals behind a train under some
circumstances.

I've seen numerous examples of this around the system and guessed that
the rationale was something similar to what you've just stated.


or only hold one signal in rear of the signal
protecting an obstruction at danger.

Why? If one red signal can protect the obstruction, what's the need
for more?

You just showed that under certain circumstances, one signal is not
enough to protect an obstruction (or at least I think you did...)

You only need one signal to proect an obstruction. Additional signals mean
that a following train can enter a platform more closely behind the one
departing whilst still maintaining a safe distance between them.

In article .com,
TheOneKEA writes
Why? If one red signal can protect the obstruction, what's the need
for more?
You just showed that under certain circumstances, one signal is not
enough to protect an obstruction (or at least I think you did...)

Um, no.

You have to make *some* assumptions when designing a signalling system.
The ones LU make a
(1) train stops will stop trains in the design distance;
(2) trains won't be exceeding the speed limit at the point they pass a
red signal in the worst situation.

Within those assumptions, one signal is all that's needed. There may be
two or more, but only one is doing the protecting.

--
Clive D.W. Feather | Home:
Tel: +44 20 8495 6138 (work) | Web: http://www.davros.org
Fax: +44 870 051 9937 | Work:
Please reply to the Reply-To address, which is: