Strong Towns's Rachel Quednau offers the rationale for Florida's Brightline. You don't build the rail line first and hope for the edge cities to follow. "The idea is that now you have manufactured a dense neighborhood where people can live without needing a car and use the rail line to commute to work." Rather, you find a way to put the rail line into a dense neighborhood, and South Florida surely qualifies. As her colleague Charles Marohn has it, "Great places need a train less than a train needs a great place. Build the place first and transit becomes the logical, inevitable next step. No more transit oriented development schemes. What we need is development oriented transit."
The challenge, where the street cars and interurbans are long gone, and the railroad routes have become bike trails, is in making that first tranche of replacement investment, replacing the bus with light rail, or upgrading a freight track for diesel multiple units, before going all in on faster full-service trains.