The Province will only agree to a grade separated alternative. And promised a "portion" underground. Attached is a link to the best map of what is expected to be the proposal.
https://urbantoronto.ca/forum/threads/eglinton-west-lrt-metrolinx.28144/page-30#post-1441159
Eglinton West has about 5 km of 14 km underground (total to the Airport). And I'm guessing it'll be cut & cover to save money.
If the DRL is above ground north of the Don 5 km of the 15 km will not be underground.
They are using a combination of at grade, above grade and below grade for Eglinton West. As so they should for the DRL where it makes sense.