The CSX line to Miami FL goes from Jacksonville FL to Orlando FL then through Winter Haven then to Okeechobee FL and then east to(through Indiantown FL) West Palm Beach FL and then south from West Palm Beach FL.
Also on the CSX line from West Palm Beach to Miami FL is the Tri-Rail commuter train.
Have you ever looked at some of the tracks along the CSX line? Some of the tracks at the stations along the CSX line look like they need immediate repair as the track pins are sticking up at an inch or more out of the crossties.
FEC runs from Jacksonville to Miami FL along the east coast of Florida, that's part of the reason for it's name. So for the All Aboard Florida there is only the need to run new track from Cocoa Beach FL to Orlando FL for FEC, maybe double track some other areas.
FEC and CSX tracks do not meet in Miami, they may be close by but don't meet up as far as I know of. Amtrak would have to diverge at Jacksonville for the Miami FL run on FEC tracks.
A few years back, almost 10 years, Amtrak had made an inaugural run on FEC tracks to Miami being led by an FEC GP-40. The project went under because of no additional federal funding for Amtrak. I wish that project would have gone through as there are more large cities on the east coast of Florida then in central Florida.
One thing that I have noticed about FEC tracks is that they get inspected and or repaired close to every two months. Also FEC uses mainly concrete crossties except at some bridges.
Lee Fritz