The fastest train currently in long-distance service is the Fuxing Hao CR400AF/BF which is currently in service at 350 km/h (220 mph). San Diego to Jacksonville

The fastest train currently in long-distance service is the Fuxing Hao CR400AF/BF which is currently in service at 350 km/h (220 mph). San Diego to Jacksonville is 3350km (2100mi) straight line. That gives a lower bound of 9.5 hours. Of course the actual path couldn't be a straight line, and the train would need to accelerate and decelerate several times for intermediate stops.