actually, i read something recently about a specially designed hydrodynamic projectile tip that vaporizes water as it comes into contact with it at speed. so, the only obstacle when it comes to firing one underwater would be isolating the electrical current from the water, as volt mentions.

If you care the keel issue is a non issue since there is no recoil to speak of with this system. And regardless of what precautions you take the life span of something like this is gonna be shorter on a sub than it would be on the deck of a ship.

I used to be a submariner, So naturally a new railgun being applied to this weapons platform sounded fun and came quite naturally.  I'm not saying its feasible, its just an entertaining thought experiment.  Also, tomahawk missiles are very expensive(as opposed to the far cheaper railgun shots) and you only have a limited, small number that you can fire(as opposed to potentially hundreds of railgun shots).  If you want to have a stealthy ship with long range long term firepower that can store a lot of railgun shots... well you have something to be feared.  Think how many merchant ships you could sink with one submarine, and think about how much the cost of your ammunition would be compared to the value of the tonnage sunk.  Honestly, it would be a weapons platform to be feared.  The potential for political plausible deniability(who fired the shot!---US---> "I don't know") would be a good sell to politicians, as this would be a weapon of war they could use to gain leverage in diplomatic discussions.  The submarine fleet is headed this way already with missiles (higher volume), the railgun submarine, if possible, would be a great addition to the capability of the US fleet.  Also, this idea is far less crazy then the fully submersible carrier that has both been extensively designed and possibly built. lol.

@ nate

you are totally correct

Not dismissing, just hadn't thought about heat generation.  I don't know the specs on this, i'm sure its going to be a lot.  I don't know much about thermodynamics aside from the basic laws, please educate me. .

Using the external hull of the submarine would be a no no, you don't want to screw with the malleable properties of the ships metal skin via high heat.  This internal compartment could be connected to the fire main system and cooled via seawater easily enough, many systems on modern submarines receive cooling this way.

@ seleucia

A tank? not so much.  Submarines used to have deck guns.  To my knowledge a torpedo tank hasn't been built, but I could be wrong.  I could see their use in river warfare, high speed torpedoes designed to put holes in small targets.  park this bad boy on a bridge and hold the entire river.  I think a 50 cal would be cheaper, and more effective.  Yet it all depends on your opponents capability.  If they have some heavily armored "tank like" river boats that could survive 50 cal fire, well then a torpedo is a great alternative.  Also, a 50 cal requires direct line of sight for accurate fire, while a well designed small torpedo could be used to shoot around river bends before opposing river craft could use any of their direct line of sight weaponry.  The advantage of having this capability on a tank is that you can move it wherever you wish. 

hmm good idea to take from the ruskies.

After all, they used to have real nuclear torpedoes for the purposes of Mutually Assured Destruction.  Vaporize a few miles of ocean water in hopes to get lucky in sinking US submarines that they couldn't track.