It appears that Miami-Dade officials are actually considering the possibility of extending Metrorail at grade. That way they don't need to build new maintenance facilities, train new staff, etc for a totally new light-rail technology.
http://www.miamiherald.com/news/loca...120723273.html
Technically, I'm not sure how the details would work... But the heavy-rail vehicles do need to run on their own right-of-way, so for e.g. the North Corridor, Metro would either need to eliminate lanes on NW 27th Ave to create a median, or widen the roadway by about 30 feet and eliminate most left turns. Businesses and residents won't be happy, and this will virtually shut down pedestrian activity along the corridor since you can't cross. Possibly the rail line would include flyovers at busy intersections to avoid congestion.
It's also unclear how officials plan to deal with the hazards of third-rail on the ground, especially in a flood-prone place like Miami. Possibly they will need to build the new sections with overhead wire, but that requires retrofitting the whole train fleet. Or they could add overhead wire to the existing network, but that's also costly and might require the replacement of station canopies.