Returning trains to the downtown train station has been a mental exercise of mine for awhile now.
I don't see how you could not do a cut and cover tunnel down the canal to bring trains to the downtown core. It's a big ditch. The new platforms can be constructed by mining like the LRT stations are now. The existence of Fallowfield station allows an electric pusher/puller to be added/removed to the train while stopped for the trip into Ottawa and through the tunnel.
Yeah, there are hurdles involved- the biggest being the fact trains can't change grade quickly. But, the idea is:
Grade down the line and tunnel under Billings Bridge, possibly cut and cover under riverdale ave. then cut and cover down the ottawa canal. The challenge is grades around the river and getting to the canal.
And the feasibility in spending millions on this, just to return the trains to the downtown.
But, if we do it. We could also integrate the train station with the Rideau Centre & LRT station to the East and the NAC / Confederation Square in the West. In my vision you could off your train and exit into Confederation Square. Or get off the LRT in the Rideau Centre and pass through the Grand Hall (or at least bellow it in an extension of it) of the train station on the way to the Confederation Square exit.
This would also free up the current train station for- DING DING DING- a new home for the Canadian Science & Tech. Centre, or a Canadian Railroading Centre.