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.

15 hours if the train didn't stop, considerably more if the practicalities of an actual rail system were in play.

Japanese high speed rail has a top speed of 200 mph but not all lines, coast to coast is 3000 miles in the US depending on where you want to start and stop.

If you take exist to be “exists as a prototype” or “exists, but only serving a short route” then you could cover ~3000 miles in 11.5 hours without stopping using the Shanghai Transrapid,[1] or 10 hours using the latest Transrapid[2] or L0 Series shinkansen.[3]

[1] https://en.wikipedia.org/wiki/Shanghai_maglev_train

[2] https://en.wikipedia.org/wiki/Transrapid

[3] https://en.wikipedia.org/wiki/L0_Series