PB blaster is my weapon of choice and others work well too but, WD40 really isn't a penetrating oil. It was designed by/for the military as a water displacement agent thus the WD (Water Displacing). As locknut mentioned, a screw extractor may have to be employed but, as a last resort and as opposed to hacksawing between the rail and post. Now that I mention it, I never did get my free extractor from this website.
Forgot to mention that there are impact screwdrivers on the market that work well too. It comes with multiple tips and you wack it with a hammer. The impact makes the head turn and there's not much that can withstand that type of simultaneous impact and twist. Good Luck.