This interesting weapon is a "8-inch Gun Railway Mount M1918. Originally this gun was mounted on a rail car, however, it was removed from the car for display in a memorial.
It was designed during the WW1 to take advantage of the large number of 8-inch disappearing guns emplaced in our coast defenses that were rapidly being outranged by the weapons found on ships. By removing them from their the disappearing carriages and mounting them on railway carriages, they could be used in France as heavy artillery during WW1. However, the war ended before they were fully fielded.
After WW1, these weapons were utilized as mobile seacoast guns by the Coast Artillery. They could be utilized at many locations along the coasts, and were relatively easy to move and prepare for firing. The railway carriages had heavy "outriggers" that would brace the carriage against the shock of the gun firing, and prevent damage to the tracks.
This particular weapon actually is the second seacoast gun to occupy this site. Until WW2, an 8-inch disappering gun was mounted at this memorial. It was scrapped during WW2, and later replaced with the current gun.
This image shows the 52nd Coast Artillery firing an 8-inch railroad gun at Fort Story, VA. Note the outriggers to bracing the railroad carriage, which is missing from the gun at Tampa.
The surviving gun can be found at the University of Tampa, in Tampa, FL.