All cross border rail service is suspended indefinitely due to the pandemic. The Cascades has substitute bus service between Seattle and Vancouver only, but not the intermediate stations. The Maple Leaf doesn't even have that.
You could take an Empire Service train to Niagara Falls, NY, taxi or uber to the Rainbow Bridge, cross as a pedestrian. Metrolinx/GO Transit has several buses a day between Niagara Falls, ON and Toronto with a transfer to train in Aldershot or Hamilton (I misremembered which).
Then you could take VIA to Ottawa. BTW, the Maple Leaf did not connect with any trains to Ottawa. It arrived too late. You would have had to layover in Toronto. You need to look at VIA's service between Ottawa and Toronto and work backwards from there to try to construct a same day connection using Empire Service and Metrolinx.
It sounds pretty arduous to me, self transfer across the border from train to bus, bus to train at Aldershot(?), train to train at Toronto.
@PVD's driving suggestion sounds both easier and quicker.