Little known fact (and I know this because my uncle was ADM (air) at the department of transport (now Transport Canada) in the early 70's), when "super-airports" hubs were looked at in the late 60's- early 70's, TC concluded that neither Toronto, nor Montreal were big enough to justify a super airport. The plan put forward to the politicians by the department was to build a single super-airport near Kingston and at the same time, build HSR connections to Toronto (1h20min), Montreal (1h15 min) and Ottawa (1h 5 min.)
But politicians got into the act: Both Toronto and Montreal had to get a super-airport. Montreal started first with Mirabel, but Toronto saw the mess it was and stopped it before they could get started , choosing to expand their current one instead.
To give you an idea of the size of the Mirabel operation: There was a meteorological event in the late 70's that closed all of the US Eastern seaboard airports North of Georgia and all the way in to this side of the Appalachians. The planes were all diverted to Mirabel. During that event, it operated at 50% capacity.