In message , Clive D. W. Feather
writes
Otherwise all those points are set to "needed" (causing them to move if
they're currently wrong). One the train passes over each set they move
back to "free" (unless another route is also holding them)
What's the chances that the points freed at Poters Bar, whist the train
was going over them, allowing the first part of the train in the right
direct and the second part by the point being able to move?
--
Clive.

In article , Clive Coleman
writes
Otherwise all those points are set to "needed" (causing them to move
if they're currently wrong). One the train passes over each set they
move back to "free" (unless another route is also holding them)
What's the chances that the points freed at Poters Bar, whist the train
was going over them, allowing the first part of the train in the right
direct and the second part by the point being able to move?

None at all, the points "fell apart" because there were strategic bolts
missing.
--
"now, the thing you type on and the window you stare out of are the same thing"

In article , Clive Coleman
wrote:
In message , Clive D. W. Feather
writes
Otherwise all those points are set to "needed" (causing them to move if
they're currently wrong). One the train passes over each set they move
back to "free" (unless another route is also holding them)
What's the chances that the points freed at Poters Bar, whist the train
was going over them, allowing the first part of the train in the right
direct and the second part by the point being able to move?

--

In article , Clive Coleman
wrote:
In message , Clive D. W. Feather
writes
Otherwise all those points are set to "needed" (causing them to move if
they're currently wrong). One the train passes over each set they move
back to "free" (unless another route is also holding them)
What's the chances that the points freed at Poters Bar, whist the train
was going over them, allowing the first part of the train in the right
direct and the second part by the point being able to move?

Where a branch line diverges from a main line, as at Hitchin, when idle
and no train is immediately expectd. the points normally return ("default" it
would be called in computer terms) the straight ahead main-line setting.

Is this a question about the Potters Bar crash? I don't think it happend like
this.

Michael Bell
--

In article , Michael Bell
writes
Where a branch line diverges from a main line, as at Hitchin, when idle
and no train is immediately expectd. the points normally return ("default" it
would be called in computer terms) the straight ahead main-line setting.

That's called "self-restoring" and it is *NOT* the normal arrangement.
Indeed, if you watch at Hitchin you can see it not happen.

There are a few places where points are made self-restoring. There are
rather more cases where points may move for a not-immediately-obvious
reason. For example, at crossovers - the links between adjacent tracks -
it is normal for both ends to move at the same time. So the points at
the end of the northbound platform at Hitchin have to be set for
Peterborough before a northbound fast train can go through, even though
it doesn't cross them. There are more complex layouts where the
relationship is less obvious, but it's there nevertheless.

--
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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In article , Clive Coleman
writes
In message , Clive D. W. Feather
writes
Otherwise all those points are set to "needed" (causing them to move
if they're currently wrong). One the train passes over each set they
move back to "free" (unless another route is also holding them)
What's the chances that the points freed at Poters Bar, whist the train
was going over them, allowing the first part of the train in the right
direct and the second part by the point being able to move?

Zero.

As well as various controls in the signalling logic (note that I said
"once the train passed, modulo misspelling), there is a separate "direct
track locking". If the track circuit covering the points is not clear,
the points are not allowed to move. The relay concerned is "slow to
rise" to allow for a momentary failure to detect the train.

This was allowed for in the investigation. There was no train movement
in the near future requiring those points to be swung, so there would
have been no reason for them to try to move.

In any case, the points were found to be correctly set and locked. The
accident happened because one of the blades broke free from the locking
mechanism underneath the train. The processes involved - and the errors
in assembly - are well understood by now; the question is *why* the
points were wrongly assembled.

--
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: