I was thinking that maybe you could have a mod that would mess with it in some way, but then again you are finding tons of ammo and mags, so I would say to just keep looking.
Just in case, here is an online item locator that someone made. It tells you where certain items can spawn.
http://cdda.sikstus.com/items.html
Edit: It looks like the chances of the railgun spawning are really low, so it could be safe to say that it is just bad luck.