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[email protected] · May 17th 11, 09:35 AM posted to uk.transport.london

In article , (Clive
D. W. Feather) wrote:

*From:* "Clive D. W. Feather"
*Date:* Mon, 16 May 2011 23:10:06 +0100

In message ,
d wrote:
The 95 stock were slugged on arrival to prevent them being able
to run
faster than the 59 stock then on the line. This was supposed to
be removed
when all the 59 stock had gone. This was then changed to "it will
be
removed when the new signalling comes in".
I don't understand the logic behind that. Surely they'd want the
trains to
have the maximum possible performance for when it can be used? eg
going up
the steep incline from archway to highgate or the long run from
east
finchley to finchley central?

The signalling may have been designed with certain assumptions in
(e.g. "a train passing Archway at 5mph won't exceed 20mph at signal
XYZ"). Changing the train characteristics could invalidate those
assumptions.

IIRC there was a collision or derailment near Watford Junction
because modern rolling stock invalidated the signalling design
assumptions.

--
Clive D.W. Feather | Home:
Mobile: +44 7973 377646 | Web: http://www.davros.org
Please reply to the Reply-To address, which is:

Because there's a maximum speed restriction on the line, drivers should
never exceed that speed and, as far as I'm aware, excessive overspeed
isn't normally taken into account (or wasn't). I suppose the existing
signalling could have a "well a driver's never going to be able to get
more than x mph" built into it.

It used to be taught that a signal was always "placed a safe braking
distance" from the train in front, based on train speed, gradient and
weight. However, the Holborn crash proved that wrong and to "safe" was
dropped and it became "placed a braking distance"!

Normally the only time there isn't the "safe" braking distance is at
station starters that have a short overlap and, in theory at least, the
rear of the train in front could be just 100 feet or so away from the red
starter. (In practice, this would only happen if the train in front has
stopped after just clearing the overlap, rather than stopping at the next
signal).

As a consequence, drivers non-stopping a station where trains normally
stop have to reduce their speed to 5mph at the starter. If the starter's
green, there will normally be a sufficient braking distance. However, the
idea is that drivers will always approach the starter as if is at danger,
rather than running full speed only to find it remains red and they go
past the red signal at speed with potentially serious consequences.

I suppose that this could be another reason for not allowing the trains to
run at their maximum power, so that trains can't approach a station any
faster than they can at the moment, but I still can't see it as a real
reason. Perhaps they're just being cautious, or perhaps it's "tidier" to
leave things as they are until the new signalling comes in.
The other thing, of course, is that the update has now taken a lot longer
than originally planned. Perhaps at the time they thought "Oh well, it'll
only be a couple of years by the time everything's finished, we'll wait
until then"!

Roger