I believe part of the RER plan is to use smaller trains running more frequently to significantly reduce dwell times (loading and unloading of passengers). Perhaps the shorter trains can more easily be double berthed at exiting Union platforms?
That is possibly part of it too (at least for some routes, in conjunction with the big bilevels). Various different documents have mentioned one or others, Metrolinx RER PDF, Union revitalization PDF, and Metrolinx 2031 plan PDF, at least one place mentioned some of following:
- Union Station Rail Corridor will be resignalled for better efficiency
- Platform 26/27 is new, built 2009. It was built before the revitalization so they could shut down a few platforms during revitalization. A few still are.
- Platform 3 track may eventually be reactivated for an eastbound service (UPX uses the track westbound).
- Platform-sharing with VIA platforms (dynamic videoboards + USRC + train traffic control centre, can make this happen)
- Electric trains with shorter dwell times and faster acceleration
- Double-berthing west of York. Three tracks can be rejigged to hold six long trains, e.g. Milton line sharing track with Richmond Hill line. Though, if using shorter trains, more double-berthing could happen (e.g. 150 meter trains instead of 300 meter trains). Some things, such as positive train control, USRC resignalling, and other safety mechanisms, as well as Transport Canada rule changes, may be needed before double-berthing is allowed to happen, but this is part of the long term plan. Then, yes, we need the underground corridor (which might or might not be combined with DRL as a 4-track underground corridor; as "Metrolinx 2031" document is very suggestive of it).
All of this combined, this will allow doubling the number of trains through Union during peak period, without doubling pedestrian density in stations or on platforms (3x larger GO concourses & 2x total width of entrance-width into Union & shorter wait times emptying plaforms faster & extra platforms from 26/27 + VIA platform sharing + double-berthing).
People don't think Union can handle it, but they're making it happen gradually. The magic of funding a professionally-engineered huge station reno and infrastructure upgrades.