It might be more convenient for passengers if Crossrail were to be
constructed for shorter, double deck, trains. They do seem to be popular
with operators in other countries for commuter services, e.g. Germany,
USA,
Australia.

Shorter trains would mean shorter walks for passengers at stations.

Don't know where you get your info, but the only city in Australia operating
double deck trains is Sydney.

Melbourne had one as a trial but it never took off and it now sits in the
railway workshops waiting for spare parts worth nearly the cost of a new
(single deck) train - so local word on the rail is that it will never work
in service again, indeed a group has already been set up for it's
preservation and everyone else has forgotten it existed (judging by the
comments on the new trains being introduced by Connex referring to them as
the first new trains since the 1980s built single deck Comengs).

Brisbane runs single deck trains in 3 and 6 car formation using 25kV AC
Perth runs 2 car trains (with some new 3 car trains being built) using 25kV
AC
Adelaide runs 1 and 2 car diesel railcars
Canberra, Darwin and Hobart don't have urban rail services.

Double deck trams and buses are another story...