In article , David of Broadway
writes
Yes, that occurred to me after posting. But do Piccadilly line trains
really have to use the side tracks? I realize that doing so can reduce
delays for eastbound passengers getting off at Acton Town, but it
doesn't actually speed up train service much, does it? One train still
has to wait for the other before leaving the station.

Actually, it can help because of the longer headways needed at stations.
Put simply, when you work out the numbers you find that the minimum
possible time between trains is longer at stations than between them.
With two platforms you can negate this to some extent because one train
can be running in to the station with the previous one still departing;
with one platform, you can't let the second train in until the first has
cleared the overlap.

Admittedly, in this case you then run into the bottleneck again at
Hammersmith, but having the lower headways at Acton gives you more
robustness.

It also helps with the District. Suppose that an eastbound train from
Heathrow is in the Piccadilly platform, with a District train from
Ealing Broadway behind a Piccadilly from Uxbridge. If the latter can get
into the station before the ex-Heathrow has left, even though it blocks
the District's platform, the latter still gets through earlier.

--
Clive D.W. Feather | Home:
Tel: +44 20 8495 6138 (work) | Web: http://www.davros.org
Fax: +44 870 051 9937 | Work:
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On Tue, 20 Mar 2007 12:34:07 +0000, Clive D. W. Feather wrote:

Yes, that occurred to me after posting. But do Piccadilly line trains
really have to use the side tracks? I realize that doing so can reduce
delays for eastbound passengers getting off at Acton Town, but it
doesn't actually speed up train service much, does it? One train still
has to wait for the other before leaving the station.

Actually, it can help because of the longer headways needed at stations.
Put simply, when you work out the numbers you find that the minimum
possible time between trains is longer at stations than between them.
With two platforms you can negate this to some extent because one train
can be running in to the station with the previous one still departing;
with one platform, you can't let the second train in until the first has
cleared the overlap.

Admittedly, in this case you then run into the bottleneck again at
Hammersmith, but having the lower headways at Acton gives you more
robustness.

It's more significant than that - driver changes take place at Acton,
which means dwell time there is often longer than at Hammersmith
(etc).

asdf wrote:
On Tue, 20 Mar 2007 12:34:07 +0000, Clive D. W. Feather wrote:

Yes, that occurred to me after posting. But do Piccadilly line
trains really have to use the side tracks? I realize that doing
so can reduce delays for eastbound passengers getting off at
Acton Town, but it doesn't actually speed up train service much,
does it? One train still has to wait for the other before
leaving the station.

Actually, it can help because of the longer headways needed at
stations. Put simply, when you work out the numbers you find that
the minimum possible time between trains is longer at stations
than between them. With two platforms you can negate this to some
extent because one train can be running in to the station with the
previous one still departing; with one platform, you can't let the
second train in until the first has cleared the overlap.

Admittedly, in this case you then run into the bottleneck again at
Hammersmith, but having the lower headways at Acton gives you more
robustness.

It's more significant than that - driver changes take place at
Acton, which means dwell time there is often longer than at
Hammersmith (etc).

Two comments on that:

1. If driver changes were properly managed, there would be no need for
longer dwell times. Some of the current changes are decidedly
leisurely, even when the new driver is already on the platform when the
train arrives.

2. In my experience, full use of the platform capacity isn't always
made, causing westbound trains to crawl through Turnham Green in the
queue to enter Acton Town. It's ludicrous that a station with extra
platform capacity should be a bottleneck.

--
Richard J.
(to e-mail me, swap uk and yon in address)