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Tom Anderson

On Fri, 4 Aug 2006, Dave Arquati wrote:

wrote:
Dave Arquati wrote:

Shared running would only start east of Turnham Green. Interlining
with the North London Line should not be very difficult. It only has
about 3 to 4 trains per hour. The Piccadilly Line is a real problem.
Perhaps the solution to that would be to have CrossRail also take over
the Rayners Lane service. This would double Piccadilly Line service to
Heathrow.

Not sure exactly what you're suggesting here. Is it:
- Crossrail via Ladbroke Grove & Shepherd's Bush to Rayners Lane via Ealing
Common and to Richmond via Gunnersbury
- District continues to run to Ealing Broadway...?

That results in four western Crossrail branches with shared use between
Gunnersbury and Richmond, between Hammersmith and Turnham Green and between
Acton Town and Ealing Common!

One of the issues raised by the Montague report was that too many
branches at each end would mean poorer reliability, as it's more
difficult to ensure that trains arrive at the core section on time for
their path - making achievement of the 24tph core service difficult.

This is all very true, and i'd agree that the suggested massive takeover
in the west would be a bad idea. However, i really can't believe that
Crossrail can only support *two* branches at each end - three should be
doable, even if not four.

In particular, if there was room for one inward train to wait on each
branch, it should be trivial - divide the hour into 30 2-minute slots,
group these into four, and allocate them one to each branch, plus a spare
(so trains go Shenfield, Dartford, Broxbourne, no train). Set up the
schedule so that each branch delivers a train at the appropriate time. If
a train is delayed, and misses its slot, it sits and waits until the next
empty slot comes round - which could be a spare slot, or a slot missed by
a delayed train on another branch. The worst-case wait would be four
minutes, if a train just missed its slot and had to wait for trains from
both other branches to go through. A slightly better arrangement might be
to group the slots into six five-slot bundles, with four trains and a
spare, so that the position of the spare slot with respect to each branch
changes; otherwise, you get worse delay behaviour on one branch.

In principle, this would work for more branches, but as you increase the
number of branches, the worst-case wait gets worse. Although, if you have
several branches missing slots, the average case might not ...

tom

--
Sapere aude!

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