There are a couple reasons (not the least of which is political will but that's a separate discussion). Jacksonville sits on a coastal plain and is almost entir

There are a couple reasons (not the least of which is political will but that's a separate discussion). Jacksonville sits on a coastal plain and is almost entirely fine sand. You simply can't build a subway in that without astronomical cost. As for rail I'm pretty sure Jacksonville already has elevated rail in some places but ground rail would be difficult for the same reasons.