On Wed, 9 Sep 2009 11:46:07 +0100, "Paul Scott"
wrote:

But the Crossrail tunnels are AFAICS UK gauge, with the track offset to one side
so that a continuous walkway can be provided. I expect double deckers won't
be possible,


That's correct. The tunnels are a nominal 6.20 metres in diameter
which precludes the use of double deck trains.


and even if gauging allowed, the dwell times would defeat 24
tph running..


You could argue that the increased capacity of double deck trains,
usually assumed to be 50% greater than a single deck train, would only
need 16 tph for the same throughput of passengers. If more than 16
tph could be operated, there would be a capacity increase over using
single deck trains.

However the capital cost of the project would be greatly increased,
with many overline structures between Shenfield/Woolwich and Reading
having to be rebuilt in addition to the much higher cost of the
Crossrail tunnels.

The French obviously thought building bigger tunnels was worthwhile,
with RER lines being built to take double deck trains. However, the
French did not need to spend huge amounts of money raising overline
structures on existing lines over which the RER trains run.