The most critical capacity issue at Union Station occurs at the concourse level, and not on the platforms. The existing ticket booths and turnstyles provide nowhere near enough capacity to accommodate the crush loads that occur during rush hour and following events at the ACC, and the pedestrian traffic heading to/from the PATH makes things worse. Fix this, and most crowding problems will be solved.
The new separate platform for northbound Yonge commuters will provide sufficient capacity at platform level for decades to come. The peak boarding patterns at Union are such that the passengers are always getting on an empty train, and therefore there's very little congestion and the dwell time is short. In the morning rush when hoards of GO passengers get on the subway, the trains have already let everyone off at points to the north. After a game at the ACC, it's so late that the trains are empty anyway. The actual dwell times are currently extremely low at Union, therefore adding a third platform down the middle is of no benefit.
Union Station may serve a high number of passengers, but it functions more like a terminal station than a transfer station. It will never have the crowding issues that Yonge and Bloor encounters, is a poor candidate for a DRL station, and won't need any more platform platform upgrades than are currently planned. Fix the concourse level, and we'll be fine.
