so dumb question here but why couldnt they use the millwood bridge and viaduct like they did with the bloor street/prince edward viaduct?
Does it not support trains running below it?
This was considered during the Transit City studies for the Don Mills LRT. There are problems. First, the bridge does not have a lower deck like the Viaduct. Here is a view of the structure from the DVP:
The understructure is the truss that holds up the deck, but it does not have voids in it for a lower subway deck. You can also see the added structure holding up the outer lanes of the bridge. When it was built they were not there, but there was extra strength in the bridge to carry a never-built streetcar line to the Leaside industrial district. The only track ever built for that extension was a short piece of double track on Pape north of the old pre-subway streetcar loop. (A sister bridge to this one is on Bathurst over the Nordheimer Ravine. It was intended to carry streetcars into Forest Hill, but that never happened. As with the Leaside Bridge, the extra strength allowed the bridge to be widened from four to six lanes.)
A further problem is that the curves to follow Pape Ave, then turn north across the bridge, then east into Overlea were considered to be too tight. The new OL trains might not have quite the same constraint, but there is still no room nor spare structural strength in the bridge itself. The only way to get a new RT line of any flavour to Thorncliffe is to cross the valley on a new bridge. A tunnel under the river is out of the question as this would make for very deep tunnels and stations at Pape/Cosburn and at Thorncliffe Park.