You only need one dude nearby with a radio (preferably in a plane) for correction of fire. And being a railroad bridge, its range and direction are going to be pretty fixed as well as being quite large, so it won't be hard to hit. Or considering 18" HE shells, hard to near-miss...
According to navweaps, shell weight for the 18"/40 was 3300 pounds, though they don't have a figure on how much HE filler there was. Japanese 46cm HE shells used a 136 pound bursting charge, while the smaller American 16" guns was about 153 pounds. I'd expect an "in between" figure of 145 pounds of HE filler being acceptable, for shore bombardment. By comparison, a modern Mk 82 carries 192 pounds of (more advanced) filler, so you'd have a blast perhaps 2/3 the strength of a single 500 pounder - but that blast is pushing over a ton and a half of steel fragments from the shell itself. #rekt