Let's hope that the stations are big enough for the trains and their customers rather than architects' egos.
The trains could in my view better be 2x5x23m cars rather than 3x4x20m cars ...
OTC
The current Crossrail plans are for '5x20m units' running in pairs, with the underground station tunnels 'as tunnelled length' capable of coping with pairs of 6x20m units, but not fully fitted out. It is this 40m expansion capacity that 'btline' is suggesting won't be provided on Crossrail, if I understand his post correctly. [This was also the policy on the TW Metro underground stations - they are all 'dug out' for 6 car trains, but these have never happened, so there is approx 40m of unfinished platform at all the underground stations, behind false walls.] Using 5x23m rather than 5x20m might be impossible at this stage dependent on tunnel curvatures and the resulting gauging issues?
However I suspect that whatever the current Crossrail spec calls for it would be daft not to use a follow on order of whatever Thameslink agree on, or a very similar spec. Using full length trains rather than pairs of units gains the wasted space used up by the intermediate cabs.
Paul