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).