In article , Jon Crowcroft
writes
By chance, it was held up behind a slow train for 5 minutes, and yet arrived on
time, this _despite_ a stop outside kings cross (bottleneck: simply
because of 4 tracks from 11 platforms to get out)

4 from 11 is about the right ratio (design standards suggest at least 2
platforms for each line entering a terminus).

Trains should be timed not to intersect; the Rules of the Plan specify
minimum margins for crossing movements in the timetable.

It's unusual to be stopped on departure; most trains depart on B or D
roads (which become the Down Fast and Slow respectively after Gasworks
Tunnel).

and coming into cambridge
(bottleneck: due to only 1 stupid platform for 3 trains requiring
extreme caution
in correct use of semaphors for 2 sets of points),

For some reason I first read that as "metaphors".

I'm not sure of your point here. If there were 3 trains in the platform,
you'd have been coupling up to one of the others, hence the dead slow
running. If not, do you mean that you had to wait for one to depart
before you could enter?

They're not semaphores in either the programming or railway senses.

Each set of points can be in one of three states: needed normal (one of
the positions), needed reverse (the other), or free (not currently
needed by anything). The signaller at Cambridge selects a route (by
pressing two buttons). The signalling logic has a list of points and
positions that that route requires, plus a list of routes that are
incompatible even though they don't conflict in their points
requirements (usually routes in the opposite direction over the same
track). If any of the points are already needed in the other position,
the route is left unset. 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).

[Clearing the signal requires other tests to be met as well; for
example, all the points must be detected as locked in the correct
position.]

At Cambridge this is done by a network of relays; current to energise
one relay passes through contacts on all those defining the appropriate
conditions.

AND a stop at hitchin (as
northbound trains to cambridge cross the fast line, there's always a
good chance
that there's some southbound GNER in the way...)

When trains are running to time, there won't be. Either the GNER was
late, you were early, or you arrived during the "pathing allowance" -
basically a scheduled delay to let another train through.

I would say that if the slow train (which the driver told us was not according
to schedule) had not been there, the cruiser could have been in cambridge in 40
minutes,

It's allowed 43 for the journey, of which 30 seconds at each end - IIRC
- is for station duties. So 42.

--
Clive D.W. Feather | Home:
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