DD double decker..................................................d'oh!! Why I didn't figure that out is beyond me. Anyway, that is where drum118 and I REALLY part company. I think DD would be a huge mistake for RER.
People keep trying to compare Paris RER with Toronto's stating that it uses DD but that is a false analogy. In Paris, RER really is regional and only used for that reason because Parisians themselves enjoy a huge Metro system that is triple the size of Toronto's. RER in Toronto is going to be much more akin to an extended subway because it will offer rapid transit to million who don't have any access to it now. This means a lot more people will be getting on/off at each station and DD trains have the worse passenger flow.
You can see this in Mel/Syd where Melbourne has the single levels subway-type trains. One would think it has lower capacity than Sydney's DD but such is not the case. This is because the dwell station times are far shorter on Melbourne's system allowing the trains to run much more frequently and offering a faster service to boot. DD trains not only have longer dwell times but also do not serve people with mobility issues well. They are crammed at the front doors and this will become more acute as our population ages.
Cities are also trying to create an environment of active mobility and DD are the anti-thesis of this. There is a reason why new buses and subways now have areas for bikes, people are using them more for part of their daily commute. Now can you imagine 5 bikes and 2 wheelchairs fighting for the small areas in front of the doors?
Also, due to this being a more subway-type system, people will be using it for shorter journeys and hence will stick around the door area if only going a couple stops. This is much like in buses and streetcars where there maybe seats available but people won't use them if they are only going a short distance and will stand right at the doors blocking everyone else. DD trains are ideal for commuter rail type service where, in the morning for instance, there is very little on/off passenger flow but rather just people getting on and then everyone getting off at one terminus station ie Union pr for something like UPX with Person as the terminus.
On a more simplistic note, but certainly not an irrelevant one, having single levels for RER and DD for GO commuter would also make it much easier to differentiate the two services for the travelling public.