As someone who's lived in both Montreal and the Washington DC area (and used Metro for commuting in both), I have to say that a mixture of raised and buried tracks are really the only way to have a practical system if you want it to work beyond the dense downtown area. Many of the more open places in metropolitan Montreal like Lachine don't have any Metro stops due to the expense of digging tunnels to there. In contrast, you can take the Washington Metro from Fairfax, VA and Rockville, MD. both well over twenty miles from downtown DC.