The Miami-Dade Busway follows the former FEC corridor as far south as there's civilization, and connects to Metrorail at its north end. Keeping that as a busway may actually be a good idea; there are a number of routes that run local in various residential and commercial areas then run express on the busway. ([
http://www.miamidade.gov/transit/librar ... ps-web.pdf Map]) It's pretty adaptable for future demand - if you get enough density south of Dadeland, you could either replace sections of the busway with Metrorail, or string wires and drop rails in the concrete and run light rail mixed with the buses.
To the northwest, the existing freight spur doesn't run through residential areas. That'll probably be best served by the existing bus grid, with potential LRT overlay later.
To the west, the CSX spur might be a viable commuter route.
To the southwest, the CSX line is very probably a viable route at least as far as Richmond West.