and finally 4. When will the tubes ETA service be expanded to the
District
line

i'd love to see it on the northern line

In article om,
TheOneKEA writes
"Bobbing" is when the signal rapidly changes from aspect to aspect
without warning or reason.

You mean "or apparent reason".

By far the commonest reason for bobbing is that one of the controlling
track circuits is being intermittently shorted out or failing open
circuit. For example, if one of the wires bonding adjacent rails is
loose, the circuit can break and re-make under the vibration of trains
on adjacent tracks.

Naturally this is a major problem because a signal could bob despite an
occupied track in front of the train; thus, if it goes to green and the
driver passes it in the correct way at speed, CRUNCH.

Bobbing *to* green is extremely unlikely, because it requires a false
feed at an appropriate point (and circuits are usually designed to
require two false feeds for a wrong-side failure). There's no obvious
situation, comparable to the one above, to cause this.

--
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:

"Chris" wrote in message
. ..

I have now had a reply from customer services....


snip

My questions a
1. Why did the only audiable announcment say "because of an earlier
incident" instead of a reason and why did the LU chap on the platform with
the white wand not do any announcments?

Apparently it is now the tubes policy to provide the shortest announcment
possible. Don't know why, we were standing there long enough to hear a more
precise announcment


2. Why couldn't the empty Ealing Broadway train which was immediately
behind another Ealing bdy train have been made into a Wimbledon one?

She completely missed my point and said sometimes trains are swapped around
and this is done to the best of the signalers ability, If that was the case
there wouldn't have been Ealing Bdy trains with 6 or so people per carriage
and then eventually a Wimbledon train with commuter style two rows of people
between the seats, body parts touching, not all fitting on.


3. Why were there no C stock trains around in either direction

She said there were. Wonder were they were - we didn't pass any after
leaving Earls Ct. There were long gaps of time when they could have come
into platform 4 aswell. Could they have all been queueing for Edgware Road?

4. Why is it always the Wimbledon branch that is most affected?
and finally 4. When will the tubes ETA service be expanded to the
District line


Apparently the whole line is always affected as badly - hmm in that case I
must have imagined the overcrowding

Clive D. W. Feather wrote:

You mean "or apparent reason".

Indeed.


By far the commonest reason for bobbing is that one of the
controlling track circuits is being intermittently shorted out
or failing open circuit. For example, if one of the wires
bonding adjacent rails is loose, the circuit can break and
re-make under the vibration of trains on adjacent tracks.


True. District Dave once described a track circuit failure involving
a broken wire; he was involved in its repair.


Bobbing *to* green is extremely unlikely, because it requires
a false feed at an appropriate point (and circuits are usually
designed to require two false feeds for a wrong-side failure).
There's no obvious situation, comparable to the one above, to
cause this.

I don't know much about the trackside arrangements for track circuits,
so I accept what you've said.

Is it possible for a wet track circuit to short out in such a way that
a green aspect could be obtained? You've stated that track circuits
require a pair of false feeds to turn green; in the case of East Putney
- Wimbledon, what would need to get wet to cause such a wrong-side
failure?

In article . com,
TheOneKEA writes
Is it possible for a wet track circuit to short out in such a way that
a green aspect could be obtained?

Extremely difficult.

The basic design of a track circuit is very simple:

Supply Relay
| | | |
| | | |
=:=+==========+= =+===========+= =+==========+= =+=======+==:==
| \---/ \---/ \---/ |
/--/ \-\
| /---\ /---\ /---\ |
=:=+==========+= =+===========+= =+==========+= =+=======+==:==

=== running rail
=:= insulated gap
= = ordinary gap between rails
=+= wire bonded to rail
|

The supply will be DC in some areas, but AC at a specific frequency on
LU. The relay will be tuned to the same frequency (so a feed from an
adjacent track circuit won't trigger it). The wheels and axles of the
train short the rails, causing the relay to de-energise. Any fault in
the wiring causes the relay to de-energise. But to get a false clear on
the relay you've got to feed significant amounts of 83 1/3 Hz (or
whatever) current into the circuit to the right of the location of the
train.

You've stated that track circuits
require a pair of false feeds to turn green;

More that "double fault" is a general principle. In-the-field circuits
are often double cut (that is, both supply and return are switched by
the controlling relay) so that a false feed or false earth doesn't
trigger it.

in the case of East Putney
- Wimbledon, what would need to get wet to cause such a wrong-side
failure?

I'd be surprised if simply having water in the wrong place would
suffice. A wiring fault would be much more likely.

Are you talking about a specific event, or just a general enquiry?

--
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:

In article , Clive D. W. Feather
wrote:
In article . com,
TheOneKEA writes
Is it possible for a wet track circuit to short out in such a way that
a green aspect could be obtained?

Extremely difficult.

The basic design of a track circuit is very simple:

Supply Relay
| | | |
| | | |
=:=+==========+= =+===========+= =+==========+= =+=======+==:==
| \---/ \---/ \---/ |
/--/ \-\
| /---\ /---\ /---\ |
=:=+==========+= =+===========+= =+==========+= =+=======+==:==

=== running rail
=:= insulated gap
= = ordinary gap between rails
=+= wire bonded to rail
|

The supply will be DC in some areas, but AC at a specific frequency on
LU. The relay will be tuned to the same frequency (so a feed from an
adjacent track circuit won't trigger it). The wheels and axles of the
train short the rails, causing the relay to de-energise. Any fault in
the wiring causes the relay to de-energise. But to get a false clear on
the relay you've got to feed significant amounts of 83 1/3 Hz (or
whatever) current into the circuit to the right of the location of the
train.


It is my understanding that 83 Hz is more complex and harder to beat than
this.

We have all seen induction motors. Sometimes they are 3 phase, but sometimes
they are single phase. A phase at right angles to the main supply is created
by passing the current through a capacitor and the rotor is rotated by
currents induced by the two magnetic fields. In railway practice, a relay
vane is lifted by the two fields, which MUST be at right angles to each
other. So, the 83 current is detected, not only because it is at the right
frequency, but ALSO because it is at right angles ("in quadrature") to a
pilot current supplied from the 83 generator. It is very difficult for a
false positive to be given.

Michael Bell
--

Clive D. W. Feather wrote:

Extremely difficult.

The basic design of a track circuit is very simple:

Supply Relay
| | | |
| | | |
=:=+==========+= =+===========+= =+==========+= =+=======+==:==
| \---/ \---/ \---/ |
/--/ \-\
| /---\ /---\ /---\ |
=:=+==========+= =+===========+= =+==========+= =+=======+==:==

=== running rail
=:= insulated gap
= = ordinary gap between rails
=+= wire bonded to rail
|

The supply will be DC in some areas, but AC at a specific frequency
on LU. The relay will be tuned to the same frequency (so a feed from
an adjacent track circuit won't trigger it).
The wheels and axles of the train short the rails, causing the
relay to de-energise. Any fault in the wiring causes the relay to
de-energise. But to get a false clear on the relay you've got to
feed significant amounts of 83 1/3 Hz (or whatever) current into
the circuit to the right of the location of the train.

Ah, I see. I'd never actually seen a proper diagram of a track circuit
before; this makes things nice and clear.


More that "double fault" is a general principle. In-the-field
circuits are often double cut (that is, both supply and return are
switched by the controlling relay) so that a false feed or false
earth doesn't trigger it.

OK.


Are you talking about a specific event, or just a general enquiry?


I'm making a general inquiry. Most of the stuff I've read from District
Line drivers states that the slightest wet down there causes massive
problems with the signalling. I was just curious as to whether or not
those problems could cause a wrong-side failure.

In article .com,
TheOneKEA writes
I'm making a general inquiry. Most of the stuff I've read from District
Line drivers states that the slightest wet down there causes massive
problems with the signalling.

This doesn't surprise me.

There's always a certain amount of current leaking between the rails
through the ballast; in effect you have to treat the circuit as having a
resistor there rather than a gap. The resistance varies *a lot* between
wet and dry conditions, and tuning the parameters of the circuit so that
this leakage doesn't look like a train is a bit of a fine art. If it's
got slightly wrong, the TC will drop out, looking to the rest of the
system like a train where no train should be. Hence problems.

The different electrical arrangements south of Putney, and the need to
interface with NR signalling, doesn't help either.

I was just curious as to whether or not
those problems could cause a wrong-side failure.

This would surprise me.

--
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:

If you have a train out of service going through Earls Court to Ealing
Common, it gets described as Ealing Broadway, that's how crap the
description system is.

Wanderingjew698 wrote:
and finally 4. When will the tubes ETA service be expanded to the

District

line


i'd love to see it on the northern line