On Tue, 31 May 2005, Brimstone wrote:

Boltar wrote:

They tried the system where trains set off at 2 minute gaps without
the hindereance of signals about 100 years ago.....too many
crashes....You might be a bit late for a meeting or dinner...but this
is safe.

I'm not suggesting ignoring signals, I'm just trying to figure out the
logic behind the system they use now. Signal has failed so go slow
across the next 2 working signals. Don't get it. Signals don't fail to
green (supposedly) so if the signal after the failed one is green why
on earth go slow past it, why not just go normal line speed? It just
seems OTT.

Because of something called an "overlap". In order to provide protection for
the next train ahead the signal is positioned before the start of the track
section it controls access to. The length of the overlap varies according to
the weight and speed of the trains plus an allowance for bad weather.

If driver's resumed normal speed after passing only one signal at clear or
caustion there might be another train or other problem withing the overlap.
Then no one gets home for tea.

According to http://www.trainweb.org/tubeprune/signalling1.htm, signals
are linked to the overlap between them and their section, as well as to
the section itself (the overlap, being part of the previous section, is
also linked to that section, of course). If that's true, there can never
be anything between a green signal and the end of its section, in the
overlap or not. Thus, it is always safe to go up to full speed at a green
signal. No?

tom

PS This is eerily reminiscent of concurrent computer programs. Things
queueing up on a critical section, regulated by wait and signal
operations. Such programs are notoriously and fiendishly difficult to
write correctly - or to read!

PPS Got as far as drawing this much of a highly entertaining diagram
before deciding text alone would do:

/----O /----O /----O
==== ==================== ==================== =======
^....^

Thought i'd include it in case anyone feels like drawing pictures and
wants somewhere to start!

tom

--
power to the people and the beats