[orulz]

Even with light rail, even in street medians, grade crossings do not necessarily mean slow. If a rail line gets full signal preemption at all crossings (not just signal priority) than it is just as fast as if it were on an elevated route, with the downside that it is somewhat more disruptive to automotive traffic.

Miami's system doesn't have any tunnels to deal with, so if third rail at ground level for new construction is out of the question, it's entirely possible that trains with dual current collection (pantographs for overhead wire and shoes for third rail) could be considered. Chicago used to have such trains for the Yellow Line.

Signal preemption at grade crossings as mentioned above would be necessary since heavy rail vehicles generally aren't designed to operate in the stop-go manner of standard at-grade (without signal preemption) LRT.