Chetoph wrote:

As Crossrail is in all likelihood going to be a regional link, as
opposed to an intercity one, are there any reasons why it could not
run double deck trains similar to those is Paris?

London Regional Metro (the private sector consortium with plans to build
a more commercially viable version of Crossrail) includes the provision
for double decker trains in its plans.

Your question implies that double deck trains are better for regional
services than intercity ones, but the reverse is the case because of the
extra time the trains would take to load and unload.

I met with some people from LRM and asked why they thought it was worth
the extra expense of bigger tunnels when double deck trains would
require longer station dwell times. I was told that the tunnels required
for double deck trains aren't much bigger than for normal trains, and
aren't much costlier (the running tunnels are a relatively small
component of the cost anyway). Furthermore CTRL completion would make
some large TBMs available. There were no plans for double decker trains
initially, but they thought it made sense to keep the option open to
introduce them if Crossrail gets too crowded after several decades.
Meanwhile, the bigger tunnels would at least provide an aerodynamic
advantage.

--
Aidan Stanger