I am certainly not a big fan of building tunnels or other things in areas that experience strong earthquakes, but it seems other people don’t mind. A tunnel from South Korea to Japan.
The idea about having a train ferry is already used in China, so I guess it is plausible. It does take quite awhile to move trains on and off such ferries (since individual cars need to be fastened to the boat, the tracks need to be lined up perfectly, etc. I don’t see North Korea giving train right of way anytime soon – unless they figure they can profit from all the goods travelling through their half of Korea. Or they can get things that have been banned in North Korea by the USA, by stealing them off train cars (iPods, certain movies, etc). I suppose if they want to build a tunnel to Japan, they might as well also build a tunnel to China. That way North Korea could be bypassed altogether.