Trying to force something like this will only make it worse by deforming parts. Patience. As suggested, use liquid wrench or WD-40 and give it time. Tonyl is right in that you have to release the wedge piece. With the bolt screwed in part way you should feel the wedge being loose. If not, you have to loosen the wedge. If you don;t know what is down there I have a pic on another thread I can dig up. Could be that for some reason the rail corroded and got stuck....the WD-40 will eventually release it. Try rocking the rail slightly to crack it loose. You also have to pull it out straight. Upward force while wiggling should eventually get it out.
Al