Two things one on railguns the other on nuclear weapons.
Railguns will be seen probably within our lifetimes, the Navy already has plans for the DDX *only thing holding em back is funding* the problem that will be overcome by these new ships is that in older ships mostly all the power created by ships goes into propulsion not leaving enough to fire a power hungry railgun the new DDX will be able to shift mostly all its power to fire the railgun.
Second Nuclear weapons are still very real and we have an ass ton the base I live on has more than I want to know. In my opinion nuclear weapons is what has kept us from going into another world war. You cant win in a nuclear war. Everybody dies, if you wanna see a very good representation of what will happen go rent "On the Beach"
The primary mission of a Trident Submarine is Deterence secodary is retaliation. They have enough power to wipe out every major city on earth 24 missiles carrying MIRV warheads I dont even know myself how many but its enough. And dont worry about safty and accidents they safeguard and perform maitnece all teh time Tridents are the most well maintained ships in the navy. You dont even wanna know how much it costs to maintain. Just to give you an idea the last submairne that had a minor collision cost you guys like 20-30 million... in parts tax money well spent :-P