Finally had opportunity to explore the abandoned tracks one September weekend with a fine group of fellow explorers. They're not such grey blockheads when you meet them in person!
First heard about it from Klausius - thanks for the recommend! Found some stuff online about it, too - an article by Brown in Torontoist.
How are they 'abandoned'? The way I understand the article: i) no more CP passenger train service and ii) no requirement for freight trains into city centre. Always fascinating how quickly things can change with shift in the economy and industry.
Sometimes walking along the abandoned tracks feels like civilization has long been gone, and you're one of the last survivors... until you get to the half-mile bridge portion and you're reminded otherwise by cars (that can see you if you're not careful) on the DVP highway.