How many people can afford taxis?
This depends entirely on how the system is designed. It will depend on how many rail lines the system has, how far apart the stations are, how often the various rail lines intersect and how many buses you have to take people back and forth to the stations. With enough airport shuttle-type buses, each one covering a very limited geographic area and enough rail stations, you could have door-to-door bus service.
And the older the population gets, the more door-to-door service would be needed. I am only 37, but already have arthritis in my knees and have trouble walking at times.
Be serious.
I once read a book by John Todd (if I remember correctly). He proposed bus shelters built around giant tanks of water that were used to grow tilapia. This would be suitable in cold climates, but why couldn't we have solar powered air conditioned bus shelters here?
And again I point out that in comparison to places like Mandarin and Orange Park, not many commuters would come from St. Augustine. There are other rail lines that would be more useful and more needed.