I am genuinely shocked this never happened.Is it practical and are there alternatives? What about putting in a walkway with easy access to the Metrorail/ Tri Rail station that is next to the existing Amtrak Miami station?
A sidewalk like this shouldn't be too hard to get land for, c.f. https://www.miamidade.gov/Apps/PA/PropertySearch/ it only goes over SFRTA (Tri-Rail) land and a public street (with the north edge of it being used for car storage in the overhead images on the county property search and Apple Maps) before connecting to the existing sidewalk on NRC (Amtrak) land.