David Wilcox wrote:
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.

The rest of the system can't cope with the extra height that would be
necessary. There were tries on the Southern in the 50s (IIRC) but they spent
so long at stations while people got on and off that they delayed the rest
of the service and were considered to be not worth the effort.