I have to give you sad news, there is none. Lots of railway, no narrow gauge. However if you go to Ottawa there is a Science and technology museum that does have a few bits of 3ft narrow gauge from Newfoundland. There oddly enough is a science center in Toronto with a Welsh Quarry locomotive which was criminally cut up so one could see how a loco works, this center also has a large Emmitt collection which is taken out at Xmas time. Any other narrow stuff is archeology as in you would have to know it had been there 50 yrs ago and want to hike to look at very little, and watch out for rattle snakes. Sadly it is also the opposite direction.
Just in case you are looking at maps and the perspective has you confused Ottawa and Montreal are both 5 hr drives from Toronto. they are about an 75 minutes from each other. Most Ontario maps are in one scale for southern Ontario and half that scale again for the north, the only way it fits on a paper fold out map. Southern Ontario Highways are multi lanes, 4 or 6 lane where many "highways" north are 2 lane with lower speed limits that can go up and down as you pass through towns.
I live close to Pearson Airport (toronto international) if you have any other questions just pm me.