"Richard J." wrote:

The point is that if the stations with the heaviest passenger flows
are in the central section where you want the greatest train
frequency, then peak trains per hour will be limited by the
increased dwell times there

There is clearly a trade off between frequency and capacity, however I
find it very hard to believe that in the Crossrail context the whole
load of a train is going to change over at each of the 6 CBD stations.
In practice it doesn't work that way on *any* system and with good
loading vestibule design on DD rolling stock, significant number of
passengers can be handled at each heavily traffic station without the
dwell time impacting the *actual track capacity* in terms of people
moved.

CityRail does it in Sydney using an all DD fleet without any
particular problems.

The dwell time issue doesn't go away just because
stations further out from the centre don't have that problem.

Dwell time is simple not the issue that you are making it out to be.

Bill Bolton
Sydney, Australia